TNITEB8ITY  OF  CALIFORNIA  PUBLICATIONS 

COLLEGE  OF  AGRICULTURE 

AGRICULTURAL  EXPERIMENT  STATION 

BERKELEY,  CALIFORNIA 


WALNUT  CULTURE 
IN  CALIFORNIA 


BY 

L.  D.  BATCHELOR 


BULLETIN  No.  332 

June,  1921 


UNIVERSITY   OF  CALIFORNIA  PRESS 

BERKELEY.  CALIFORNIA 

1921 


David  P.  Barrows,  President  of  the  University. 

EXPERIMENT  STATION  STAFF 

HEADS    OF    DIVISIONS 

Thomas  Forsyth  Hunt,  Dean. 

Edward  J.  Wickson,  Horticulture  (Emeritus). 

Walter  Mulford,  Forestry,  Director  of  Resident  Instruction. 

C.  M.  Haring,  Veterinary  Science,  Director  of  Agricultural  Experiment  Station. 

B.  H.  Crocheron,  Director  of  Agricultural  Extension. 
Hubert  E.  Van  Norman,  Vice-Director;  Dairy  Management. 

James  T.  Barrett,  Plant  Pathology,  Acting  Director  of  Citrus  Experiment  Station. 
William  A.  Setchell,  Botany. 
Myer  E.  Jaffa,  Nutrition. 
Ralph  E.  Smith,  Plant  Pathology. 
John  W.  Gilmore,  Agronomy. 
Charles  F.  Shaw,  Soil  Technology. 

John  W.  Gregg,  Landscape  Gardening  and  Floriculture. 
Frederic  T.  Bioletti,  Viticulture  and  Fruit  Products. 
Warren  T.  Clarke,  Agricultural  Extension. 
John  S.  Burd,  Agricultural  Chemistry. 
Charles  B.  Lipman,  Soil  Chemistry  and  Bacteriology. 
Ernest  B.  Babcock,  Genetics. 
Gordon  H.  True,  Animal  Husbandry. 
Fritz  W.  Woll,  Animal  Nutrition. 
W.  P.  Kelley,  Agricultural  Chemistry. 
H.  J.  Quayle,  Entomology. 
Elwood  Mead,  Rural  Institutions. 
H.  S.  Reed,  Plant  Physiology. 
L.  D.  Batchelor,  Orchard  Management. 
J.  C.  Whitten,  Pomology. 
*Frank  Adams,  Irrigation  Investigations. 

C.  L.  Roadhouse,  Dairy  Industry. 
R.  L.  Adams,  Farm  Management. 

F.  L.  Griffin,  Agricultural  Education. 
John  E.  Dougherty,  Poultry  Husbandry. 
W.  B.  Herms,  Entomology  and  Parasitology. 
L.  J.  Fletcher,  Agricultural  Engineering. 
Edwin  C.  Voorhies,  Assistant  to  the  Dean. 

Citrus  Experiment  Station  Staff 

J.  T.  Barrett,  Acting  Dean  and  Director. 

W.  P.  Kelley,  Professor  of  Agricultural  Chemistry. 

H.  J.  Quayle,  Professor  of  Entomology. 

H.  S.  Reed,  Professor  of  Plant  Physiology. 

L.  D.  Batchelor,  Professor  of  Orchard  Management. 

H.  S.  Fawcett,  Professor  of  Plant  Pathology. 

R.  S.  Vaile,  Assistant  Professor  of  Orchard  Management. 

A.  R.  C.  Haas,  Assistant  Professor  of  Plant  Pathology. 

E.  T.  Bartholomew,  Assistant  Professor  of  Plant  Pathology. 

C.  O.  Smith,  Research  Associate  in  Plant  Pathology. 

E.  E.  Thomas,  Research  Associate  in  Agricultural  Chemistry. 
H.  B.  Frost,  Research  Associate  in  Plant  Breeding. 

F.  F.  Halma,  Research  Associate  in  Plant  Physiology. 
J.  G.  Surr,  Assistant  in  Orchard  Management. 
Hugh  Knight,  Assistant  in  Entomology. 

S.  M.  Brown,  Assistant  in  Chemistry. 

A.  B.  Cummins,  Assistant  in  Agricultural  Chemistry. 


*  In  cooperation  with  office  of  Public  Roads  and  Rural  Engineering,   U.  S.  Department  of 
Agriculture. 


WALNUT   CULTURE    IN   CALIFORNIA1 

By  L.  D.  BATCHELOR 


CONTENTS 

PAGE 

General  Review  of  the  Walnut  Industry 142 

Business  prospects  of  the  industry 143 

Length  of  life  of  walnut  trees 143 

Profitable  bearing  of  young  trees 145 

Climatic  Requirements 146 

Frost  injury 146 

Heat  injury 147 

Soil  Requirements 148 

Depth  and  character  of  soil 148 

Drainage  and  alkali  injury 149 

Water  Supply........ 150 

Amount  of  irrigation  water  necessary 150 

Rainfall  in  walnut-growing  sections  151 

Irrigation  water  of  good  quality  essential 152 

Varieties 153 

Description  of  varieties 155 

Placentia 155 

Eureka 156 

Ehrhardt 159 

Payne ' 162 

Concord 162 

Franquette 164 

Rootstocks  and  Choice  of  Nursery  Trees 165 

Choice  of  rootstock 165 

Method  of  propagation 166 

Requirements  for  good  nursery  trees 166 

Starting  the  Young  Orchard 166 

Laying  out  the  orchard 166 

Care  of  trees  before  planting 168 

Planting  the  young  orchard ' 170 

Training  young  trees 170 

Values  of  orchards  and  lands 175 

Culture : 176 

Soil  management 176 

Covercropping 180 

Irrigation 186 

Intercropping 196 

Fertilization 201 

Pruning 202 

Insects  and  Disease  Pests 203 

Codling  moth  and  aphis 203 

Red  spider 204 

Walnut  blight 205 

Melaxuma 206 

Winter-injury  or  Die-back 208 

Harvesting,  Curing,  Packing  and  Cost  of  Production 210 

Harvesting 210 

Washing  and  curing 212 

Packing 214 

The  standard  for  first-grade  nuts 216 

Selling  the  crop 216 

Income  to  be  derived  from  California  walnuts 217 

i  Paper  No.  82,  University  of  California,  Graduate  School  of  Tropical  Agricul- 
ture and  Citrus  Experiment  Station,  Riverside,  California. 


142  UNIVERSITY    OF    CALIFORNIA — EXPERIMENT    STATION 


PREFACE 

The  present  bulletin  represents  the  results  of  general  observations 
and  specific  investigations  of  the  writer  and  his  associates.  The 
subject  matter  and  the  method  of  presentation  have  been  chosen  in 
response  to  the  numerous  inquiries  relating  to  general  walnut  culture, 
which  have  been  received  during  the  past  six  years. 

The  writers'  thanks  are  extended  to  Mr.  Carlyle  Thorpe,  Professor 
F.  T.  Bioletti,  and  Dr.  H.  S.  Reed  for  their  careful  criticism  of  the 
manuscript,  and  to  Mr.  D.  C.  Wylie,  of  the  California  Walnut 
Growers'  Association,  for  his  able  assistance  in  gathering  much  of 
the  general  information  in  the  walnut  groves  and  among  the  walnut 
growers  in  various  parts  of  California.  Use  has  been  made  of  the 
Experiment  Station  Bulletin  231,  by  Professor  Ralph  Smith,  and  of 
"The  California  Walnut,"  by  Mr.  W.  T.  Webber  and  Mr.  W.  E. 
Goodspeed. 

For  a  more  detailed  discussion  of  the  several  ramifications  of  this 
subject,  such  as  history,  nursery  propagation,  and  the  description  of 
rarely  found  varieties  and  diseases,  the  reader  is  referred  to  Smith, 
R.  E.,  Walnut  Culture  in  California,  Univ.  of  Calif.  Bull.  231,  1912, 
and  also  to  Lake,  E.  R.,  The  Persian  Walnut  Industry  of  the  United 
States,  U.  S.  Dept.  of  Agr.,  Bur.  Plant  Ind.,  Bull.  254,  1913. 


GENERAL    REVIEW    OF    THE    WALNUT    INDUSTRY 

The  Persian  walnut  (Juglmis  regia)  or,  as  it  is  more  commonly 
known,  the  English  walnut,  may  be  seen  growing  in  nearly  every 
county  in  the  state  of  California.  The  commercial  production  of  nuts, 
however,  is  centered  mainly  in  southern  California  in  the  following 
four  counties,  mentioned  in  the  order  of  importance :  Los  Angeles, 
Orange,  Ventura,  and  Santa  Barbara.  Riverside  and  San  Bernar- 
dino counties  have  a  considerable  area  of  young  groves;  and  in  the 
central  part  of  the  state  walnut  culture  is  gradually  becoming  im- 
portant in  the  following  counties :  Santa  Clara,  San  Joaquin,  Contra 
Costa,  Napa,  and  Sonoma. 

Figure  1  shows  a  map  of  the  state  and  the  relative  importance 
and  distribution  of  the  crop  in  the  several  counties. 

At  the  present  writing,  there  are  approximately  85,000  acres  of 
walnut  trees  of  various  ages  within  the  state.  The  annual  production 
for  the  state  during  the  last  ten  years  may  be  seen  from  the  following 
table : 


Bulletin   332]  WALNUT   CULTURE   IN   CALIFORNIA  143 


TABLE  I 

Annual  Production  of  Walnuts  in  California 
(Expressed  in  pounds) 

1911 25,000,000  1916 29,200,000 

1912 22,500,000  1917 33,000,000 

1913 22,700,000  1918 40,231,000 

1914 17,800,000  1919 56,200,000 

1915 29,650,000  1920 43,000,000 

BUSINESS    PROSPECTS    OF    THE    INDUSTRY 

The  present  condition  of  the  industry  indicates  that  walnuts  may 
continue  to  be  one  of  the  most  staple  and  at  the  same  time  most  profit- 
able crops  which  can  be  grown  on  land,  and  in  locations,  adapted  to 
this  crop.  The  annual  consumption  of  walnuts  in  the  United  States 
has  ranged  between  60,000,000  and  70,000,000  pounds  annually  during 
the  past  five  years,  and  has  increased  more  than  30  per  cent  since 
1909.  Nuts,  which  were  formerly  looked  upon  as  a  holiday  luxury, 
are  becoming  more  and  more  a  food  to  be  used  the  year  through  in  the 
average  household.  This  increase  and  wider  use  of  the  walnut  has 
been  accompanied  by  an  increase  in  importation  exceeding  our  own 
average  production,  and  a  gradual  increase  in  the  prices  paid  to  the 
California  grower.  It  may  thus  be  inferred  that  the  business  of  wal- 
nut culture  in  California  is  not  likely  to  suffer  from  the  results  of 
over-production  in  the  near  future.  It  is  difficult  to  forecast  future 
economic  conditions  relating  to  the  production  and  sale  of  any  par- 
ticular crop.  Nevertheless,  many  of  the  older  walnut  growers,  who 
have  known  this  industry  from  its  early  days,  believe  that  the  planting 
of  a  walnut  grove  at  present  is  as  safe  an  investment  as  it  has  been  at 
any  time  during  the  development  of  the  industry. 

LENGTH    OF    LIFE    OF   WALNUT   TREES 

The  limit  of  the  profitable  length  of  life  of  a  walnut  grove  cannot 
be  told  from  experiences  in  California.  Provided  the  soil,  climatic, 
and  water  conditions  are  well  suited  to  this  crop,  the  trees  may  con- 
tinue to  thrive  and  produce  satisfactory  crops  for  a  long  period,  which 
is  difficult  to  limit.  Some  observers  have  expressed  opinions  that 
walnut  groves,  under  favorable  conditions,  may  continue  to  be  a  satis- 
factory source  of  revenue  until  the  trees  are  from  50  to  200  years  old. 
Some  of  the  oldest  walnut  groves  in  Ventura  County  are  the  most 
profitable  and  productive  in  the  state.  Notable  examples  of  such  old, 
yet  productive  groves,  are  the  J.  C.  Daly  grove  of  20  acres  planted 
in  1881 ;  18  acres  of  the  J.  M.  Sharp  grove  planted  in  1887 ;  and  the 


144 


UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 


T.  A.  Kelsey  grove  of  26  acres  planted  in  1888.  The  last  mentioned 
grove  has  only  12  trees  to  the  acre,  planted  60  feet  apart  each  way, 
and  is  considered  by  many  to  be  one  of  the  best  walnut  properties  in 
the  state  of  California.     The  trees,  at  their  present  age  of  34  years, 


Fig.  1.     (From  < '  The  California  Walnut. ") 

show  no  apparent  deterioration.  This  grove  has  produced  an  average 
annual  tonnage  of  1941  pounds  of  nuts  per  acre  during  the  past  four 
years.  Groves  in  which  the  trees  are  planted  60  feet  each  way  may 
be  expected  to  grow  longer  without  deterioration  than  groves  where 
the  trees  are  planted  closer  and  thus  severely  compete  with  each  other. 


BULLETIN   332]  WALNUT   CULTURE  IN    CALIFORNIA  145 

PROFITABLE    BEARING    OF    YOUNG    TREES 

The  young  walnut  grove  may  be  expected  to  bear  profitable  crops 
by  the  time  the  trees  are  6  to  10  years  old,  depending  upon  the  variety, 
the  number  of  trees  planted  per  acre,  and  the  natural  conditions 
surrounding  the  grove. 

A  grove  of  almost  50  acres  of  the  Placentia  variety  in  Orange 
County  produced  an  average  of  44  pounds  of  ungraded  nuts  per  tree 
in  its  ninth  year.  As  the  trees  are  60  feet  apart  this  represents  528 
pounds  per  acre.  During  the  first  10  or  12  years  this  might  well  have 
been  double  planted  and  thus  produced  approximately  1056  pounds 
per  acre  the  ninth  year. 

In  1918  a  block  of  60  trees  of  the  Chase  variety,  on  a  heavy  clay 
loam  soil  in  the  Puente  district,  produced  a  profitable  crop  of  35 
pounds  per  tree  in  the  sixth  year.  The  records  of  these  groves  and 
of  a  grove  of  38  Ehrhardt  trees  are  shown  in  Table  II. 

In  considering  this  table  one  should  keep  in  mind  that  each  of  these 
plantations  is  growing  under  the  best  natural  and  cultural  conditions, 
and  is  thus  without  doubt  considerably  above  the  average.  In  pro- 
jecting a  business  enterprise,  the  yields  shown  may  be  looked  upon 
as  approaching  the  maximum  possibilities  and  should  therefore  be 
discounted  25  to  50  per  cent  to  reach  a  figure  which  may  reasonably 
be  expected  to  prevail  under  average  conditions.  Again,  the  figures 
here  presented  are  of  little  value  in  making  comparisons  between  the 
three  varieties  in  question,  as  no  two  varieties  were  of  the  same  age 
the  same  season. 

TABLE  II 

Average  Yearly  Production  per  Tree  of  Young  "Walnut  Groves 

Variety 


r 
Placent 

A 

ia 

Chase 

Ehrhs 

irdt 

Year 

r 

Season's 
growth 

Yield 
in  lbs. 

Season's              Yield 

growth               in  lbs. 

(Ungraded  nuts 

at  harvest) 

r                                             y 
Season's                 Yield 
growth                 in  lbs. 
(Ungraded  nuts 
at  harvest) 

1915 

7th 

22f 





10th 

80 

1916 

8th 

27f 

.... 

.... 

11th 

76 

1917 

9th 

44* 

5th 

18 

12th 

79 

1918 

10th 

43* 

6th 

35 

13th 

81 

1919 

11th 

104* 

7th 

72 

14th 

117 

1920 

12th 

82* 

8th 

43 

15th 

98 

*  Pounds  of  uncured  and  ungraded  nuts  at  harvest, 
t  Pounds  of  cured  and  ungraded  nuts  at  harvest. 


146  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

Although  the  cultural  and  natural  conditions  in  all  these  groves 
were  favorable,  the  plantings  are  several  miles  apart  and  conditions 
were  not  identical.  Certain  seasons  are  particularly  favorable  to  crop 
production.  The  year  1919  was  characterized  by  especially  abundant 
crops  of  walnuts.  These  were  followed  in  many  cases  by  lighter  crops 
in  1920,  although  the  young  trees  were  one  year  older  and  had  a 
greater  bearing  surface. 

CLIMATIC    REQUIREMENTS 

The  chief  climatic  limitations  of  the  walnut  are  frosts  in  spring 
and  fall,  and  extreme  heat  in  summer. 

FROST  INJURY 

Low  frosty  sites  should  be  avoided  for  walnuts.  A  temperature 
of  even  2  or  3  degrees  below  freezing  (32°  F.)  will  usually  kill  a  large 
percentage  of  the  young  walnut  flowers,  if  they  are  just  starting  to 
bloom.  During  the  season  of  1920,  late  spring  frosts  during  the  second 
week  of  April  reduced  the  walnut  crop  to  a  great  extent  in  several 
of  the  inland  valleys.  In  one  instance  at  least,  the  absolute  minimum 
recorded  by  a  reliable  thermometer  was  only  29°  F.,  yet  it  was  esti- 
mated that  50  per  cent  of  the  crop  was  destroyed.  During  this  period 
apricots  and  the  earlier  blooming  cling  peaches  in  adjacent  orchards, 
were  about  the  size  of  green  peas,  and  experienced  little  or  no  injury. 

During  the  morning  of  April  6,  1921,  a  minimum  temperature  of 
27°  F.  was  experienced  by  a  seedling  walnut  grove  in  which  the 
majority  of  the  trees  were  in  full  bloom.  This  temperature  prevailed 
for  about  an  hour  and  apparently  destroyed  nearly  the  whole  potential 
crop,  except  in  the  tops  and  centers  of  the  trees,  where  an  occasional 
cluster  of  blooms  escaped  injury. 

As  a  general  rule,  the  late  blooming  varieties,  such  as  the  Eureka 
and  Concord,  may  be  expected  to  escape  injury  from  the  late  spring 
frost  more  consistently  than  the  earlier  blooming  soft-shell  types. 
During  the  spring  of  1915,  however,  walnut  twig  tips  and  young  nuts 
were  killed  during  the  first  week  in  May  on  some  of  the  low  sites  in 
Los  Angeles  County.  With  a  frost  as  late  as  May  the  late  blooming 
varieties  may  suffer  more. injury  than  the  earlier  ones,  as  the  young 
nuts  (one  half -inch  or  more  in  diameter)  of  the  early  blooming  varie- 
ties, at  this  time,  will  stand  more  cold  than  the  blossoms  of  the  late 
varieties.  The  late  bloomer  does  not,  therefore,  always  escape  late 
spring  frosts.  It  is  much  better  to  avoid  planting  walnuts  on  land 
which  is  considered  too  frosty  for  peaches  during  the  majority  of 


Bulletin  332]  WALNUT  CULTURE  IN  CALIFORNIA  147 

seasons.  The  varieties  of  the  soft-shell  type  bloom  about  the  same 
time  as  the  Bartlett  pear  and  are  not  nearly  so  resistant  to  frost 
injury  of  the  blooms. 

Early  fall  frosts  also  cause  injury,  especially  to  young  shoot 
growth  ;  though  this  injury  may  not  be  apparent  to  the  casual  observer 
until  the  following  spring,  when  the  tree  fails  to  leaf  out  on  the  major 
portion  of  the  past  season's  growth.  This  type  of  injury  is  more  fully 
treated  under  the  subject  of  die-back.  It  may  suffice  to  explain  here 
that  early  fall  frosts  prematurely  stop  the  final  ripening  process  of  the 
twigs  by  killing  the  leaves.  Such  injured  leaves  will  usually  fall  off 
within  a  week  after  the  frost  injury,  whereas  they  might  otherwise 
have  functioned  normally  a  month  or  six  weeks  longer.  The  bare, 
immature  twigs  dry  out  rapidly  and  sunburn  on  the  south  side  during 
the  winter  months,  killing  back  several  inches,  or  possibly  several  feet 
to  the  more  mature  wood,  which  has  been  able  to  withstand  the 
abnormal  insolation. 

HEAT  INJURY 

The  chief  injury  to  the  walnuts  during  the  extreme  hot  weather 
of  the  summer  months  is  the  sunburning  of  nuts  on  the  outsides,  tops, 
and  south  sides  of  the  trees.  Such  burned  nuts  usually  become 
"blanks"  if  the  injury  occurs  during  June  or  July.  If  the  sunburn- 
ing takes  place  when  the  nuts  are  more  fully  developed,  in  August, 
they  may  be  salvaged  as  culls  with  a  portion  of  the  kernels  edible.  In 
mild  cases  of  sunburn  injury,  a  small  part  of  the  husk  may  stick  to 
the  shell  or  may  only  stain  it,  causing  the  nut  to  be  graded  out  as  a 
cull.  The  kernels  of  such  nuts  may  be  of  first  quality,  although  it  is 
likely  that  there  will  occur  a  high  percentage  of  amber  and  black 
kernels  and  partly  shriveled  and  mouldy  meats  among  this  class  of 
nuts.  Nuts  on  trees  growing  on  a  deep  silt  soil  withstand  the  extreme 
heat  without  burning  much  better  than  nuts  on  trees  growing  on  a 
shallow  soil  with  less  moisture  available.  Sandy  soils  must  be  fre- 
quently irrigated  in  the  hot  sections  if  the  walnuts,  are  to  avoid  serious 
injury  by  sunburning.  At  a  maximum  temperature  of  100°  F.  in  the 
shade,  accompanied  by  low  humidity,  walnuts  which  are  exposed  to 
the  direct  rays  of  the  sun  may  be  expected  to  sunburn.  This  tempera- 
ture, or  higher,  occurs  in  many  of  the  inland  valleys,  15  to  30  days 
during  the  summer  months.  Under  such  conditions  the  crop  is  always 
injured  by  sunburn.  Regardless  of  this,  the  planting  of  walnuts  is 
extending  more  and  more  inland.  The  planting  of  oranges  is  restrain- 
ing the  development  of  walnut  groves  on  the  coastal  sections.  At  the 
present  time  walnut  groves  have  not  become   as  productive   in  the 


148  UNIVERSITY    OF    CALIFORNIA — EXPERIMENT    STATION 

inland  valleys,  which  experience  frequent  summer  days  with  maximum 
temperatures  of  100°  F.  and  over,  as  they  have  been  nearer  the  coast, 
where  the  weather  is  characterized  by  a  smaller  daily  range  and  a 
lower  maximum  temperature.  Varieties  especially  selected  for  the 
hotter  sections  may  tend  to  correct  this  difficulty  in  the  future.  In  the 
past,  most  of  the  walnuts  planted  inland  have  been  of  the  same  varie- 
ties as  those  which  were  originally  selected  for  the  coastal  regions. 
Varietal  adaptation  is  more  fully  treated  under  the  heading  of 
varieties. 

At  present  walnut  culture  is  most  successful  in  the  sections  of  the 
state  which  are  characterized  by  a  long  growing  season  free  from 
frosts,  where  the  daily  range  in  temperature  is  not  great,  where  the 
absolute  maximum  seldom  goes  above  100°  F.,  and  where  winter  tem- 
peratures are  moderate  and  the  humidity  is  high,  especially  during 
the  summer.  Sudden  changes  in  temperature,  such  as  occur  in  the 
desert  regions,  seem  decidedly  detrimental  to  the  growth  of  the  walnut 
tree.  The  localities  where  favorable  climatic  conditions  occur  is  indi- 
cated by  the  fact  that  the  largest  walnut  shipping  centers  in  the  state 
are  Santa  Ana,  Whittier,  Puente,  Saticoy,  and  Santa  Barbara. 

When  the  tree  is  fully  dormant,  the  absolute  minimum  tempera- 
tures of  winter,  during  January  and  February,  apparently  have  very 
little  to  do  with  its  welfare.  This  is  especially  true  of  the  French 
varieties.  Trees  of  the  Mayette  and  Franquette  varieties  have  grown 
for  some  years  without  severe  winter  injury  along  the  west  side  of 
the  Wasatch  Mountains  north  of  Salt  Lake  City,  Utah,  where  the 
minimum  temperature  occasionally  was  5°  below  zero  (Fahrenheit) 
during  the  winters  of  1913-14  and  1914-15. 

SOIL    REQUIREMENTS 

DEPTH  AND  CHARACTER  OF  SOIL 

The  success  of  walnut  culture  is  dependent  upon  favorable  soil 
conditions  to  a  greater  extent  than  that  of  many  other  tree  crops.  A 
well-drained  deep  silt  loam,  containing  an  abundance  of  organic 
matter,  free  from  a  high  or  fluctuating  water  table,  hardpan,  sandy 
subsoil,  and  ' '  alkali, ' '  is  essential  for  a  first-class  walnut  grove.  Shal- 
low soils  underlaid  by  a  hardpan  or  an  impervious  clay  stratum  within 
5  or  6  feet  from  the  surface  will  produce  only  a  short-lived,  second- 
rate  walnut  grove.  Such  groves  are  frequently  stunted  in  growth, 
subject  to  yellowness,  and  are  poor  producers. 

There  are  a  few  moderately  successful  groves  growing  in  fine  silt 
soil  underlaid  with  fine  sand  within  4  or  5  feet  from  the  surface, 
but  such  properties  require  the  greatest  skill  in  planning  the  cultural 


Bulletin   332]  WALNUT   CULTURE   IN    CALIFORNIA  149 

and  irrigation  practice  and  should  certainly  be  avoided  by  the  beginner 
in  walnut  growing. 

Very  light  sandy  soils  are  usually  unsatisfactory,  producing  slow- 
growing,  stunted,  poor-yielding  trees.  On  such  soils  the  nuts  are 
very  subject  to  sunburning.  One  should  not  judge  the  soil  conditions 
entirely  by  the  surface  soil.  The  nature  of  the  subsoil  may  be  of  great 
importance.  There  are  first-class  walnut  groves  in  the  El  Monte 
district  growing,  apparently,  in  a  very  light  sandy  soil.  The  soil 
changes,  however,  in  some  instances  3  or  4  feet  from  the  surface, 
grading  into  a  silt  loam  which  seems  to  be  an  ideal  soil  for  walnut 
roots. 

Very  heavy  adobe  soils  or  silts  underlaid  by  adobe  are  not  best 
suited  to  walnut  groves.  The  trees  on  such  soils  are  usually  stunted, 
making  very  little  growth  and  fruit  wood  in  the  tops  when  mature. 
These  soils  are  difficult  to  cultivate,  and  it  is  difficult  to  obtain  a  deep 
penetration  of  the  irrigation  water  which  is  so  essential  to  good  walnut 
culture.  Such  soils  frequently  contain  amounts  of  alkali  harmful  to 
walnuts,  and  their  leaching  by  any  system  of  irrigation  is  very  difficult. 

DRAINAGE   AND   ALKALI   INJUEY 

A  water  table  within  9  or  10  feet  from  the  surface  of  the  soil 
may  make  it  impossible  to  maintain  a  profitable  walnut  grove.  Most 
of  the  high  water  tables  in  arid  regions  are  characterized  by  carrying 
more  or  less  "alkali"2  in  solution.  The  mere  presence  of  water  within 
9  or  10  feet  of  the  surface  is  usually  only  part  of  the  difficulty, 
the  alkali  present  is  the  greatest  concern,  for  the  deep-rooted  trees 
obtain  a  portion  of  their  water  from  this  water  table  and  may  be 
injured  by  the  salts  therein,  even  though  there  is  a  seemingly  sufficient 
layer  of  good  soil  above  the  water.  There  occurs  an  instance  in 
Orange  County  where  an  old  grove  has  been  practically  ruined  by 
a  rise  in  the  water  table  to  within  only  nine  feet  of  the  surface.  This 
water  contains  700  parts  per  million  of  salts.  The  water  rises  approxi- 
mately 6  feet  by  capillary  action  in  the  soil;  thus  there  is  only  a 
3-foot  depth,  or  one-third  of  the  soil,  above  the  standing  water,  which 
is  not  impregnated  with  salts.  Several  instances  were  noted  in  Ven- 
tura County  during  the  fall  of  1919  where  walnut  trees  were  severely 
injured  by  the  presence  of  an  "alkali"  ground  water  9  to  13  feet 
from  the  surface.  When  the  trees  were  planted  20  years  ago  in  this 
district,  the  water  was  at  least  30  feet  from  the  surface.  It  is  difficult 
to  place  a  definite  safe  boundary  beyond  which  injury  from  a  high 


2  For  a  discussion  of  alkali  in  the  irrigation  water  see  page  152. 


150  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

water  table  is  likely  to  occur.  Apparently  quite  as  much  depends 
upon  the  nature  of  the  water  as  upon  the  actual  depth  from  the 
surface,  within  the  limitation  of  the  root  zone.  The  rainfall,  irrigation 
practice,  and  nature  of  the  soil,  and  the  irrigation  water  may  also 
greatly  influence  the  limitations,  for  walnut  culture,  of  land  with  a 
high  water  table. 

Rarely  there  is  found  an  exception  to  the  rule  of  injury  resulting 
from  a  high  water  table.  There  is  one  notable  example  in  Santa 
Barbara  County  where  an  orchard  is  sub-irrirgated  by  a  natural  flow 
of  water  from  the  hills.  Judging  from  the  appearance  of  the  trees,* 
this  ground  water  contains  no  (or  an  exceedingly  small  amount  of) 
"alkali,"  a  condition  which  is  exceptional  to  the  usual  shallow  water 
tables  in  irrigated  areas  of  the  arid  west. 

WATER    SUPPLY 

In  most  of  the  California  walnut  districts,  irrigation  is  necessary. 
The  amount  of  water  to  be  applied  and  the  seasons  of  application 
naturally  depend  upon  the  rainfall,  the  amount  of  evaporation,  the 
nature  of  the  soil,  and  other  factors.  The  walnut  tree  can  not  be 
expected  to  yield  profitable  crops  unless  furnished  with  an  abundant 
supply  of  water  during  the  entire  year. 

AMOUNT  OF  IRRIGATION  WATER  NECESSARY 

From  12  to  24  acre-inches  per  acre3  of  irrigation  water  is  usually 
applied  to  bearing  walnut  groves,  varying  according  to  some  of  the 
conditions  related  in  the  preceding  and  the  following  paragraphs. 

3For  those  not  familiar  with  the  various  methods  of  measuring  water,  the  fol- 
lowing explanation  and  table  is  offered  converting  miners'  inches  to  acre  inches. 
An  acre  inch  of  water  is  the  quantity  of  water  required  to  cover  one  acre  to  a 
depth  of  one  inch. 

50  California  "miners'  inches"  =  1  cubic  foot  per  second. 

1  cubic  foot  per  second  =  7.48  gallons  per  second. 

Table 
Miners'  inches  to  acre  inches. 


Miners 

'  Inches 

Hrs. 

1 

2 

3 

4 

5 

6 

7 

8 

9 

10 

1 

.0198 

.0396 

.0594 

.0792 

.0990 

.1188 

.1386 

.1584 

.1782 

.1980 

2 

.0396 

.0792 

.1188 

.1584 

.1980 

.2376 

.2772 

.3168 

.3564 

.3960 

3 

.0594 

.1188 

.1782 

.2376 

.2970 

.3564 

.4158 

.4752 

.5346 

.5940 

4 

.0792 

.1584 

.2376 

•.3168 

.3960 

.4752 

.5544 

.6336 

.7128 

.7920 

5 

.0990 

.1980 

.2970 

.3960 

.4950 

.5940 

.6930 

.7920 

.8910 

.9900 

•s 

PI 

6 

.1188 

.2376 

.3564 

.4752 

.5940 

.7128 

.8316 

.9504 

1.0892 

1.1880 

7 

.1385 

.2772 

.4158 

.5544 

.6930 

.8316 

.9702 

1.1088 

1.2474 

1.3860 

<v 

8 

.1584 

.3168 

.4752 

.6336 

.7920 

.9504 

1.1088 

1.2872 

1.4256 

1.5840 

2 

9 

.1782 

.3564 

.5346 

.7128 

.8910 

1.0692 

1.2474 

1.4256 

1.6038 

1.7820 

< 

10 

.1980 

.3980 

.5940 

.7920 

.9900 

1.1880 

1.3860 

1.5840 

1.7820 

1.9800 

11 

.2178 

.4356 

.6534 

.8712 

1.0890 

1.3038 

1.5246 

1.7424 

1.9602 

2.1780 

12 

.2376 

.4752 

.7128 

.9504 

1.1880 

1.4256 

1.6632 

1.9008 

2.1384 

2.3760 

BULLETIN   332]  WALNUT   CULTURE  IN    CALIFORNIA  151 

The  amount  will  vary  with  the  nature  of  the  soil,  as  an  open  sandy 
soil  requires  more  water  to  produce  equivalent  crops  than  a  compact 
silt  loam.  Closely  planted  and  old  bearing  groves  will  require  more 
water  than  young  groves  or  those  with  fewer  trees  to  the  acre,  other 
things  being  equal. 

The  rainfall  varies  from  season  to  season  and  in  some  years  it  may 
be  necessary  to  apply  50  per  cent  more  water  than  in  others.  There 
is  considerable  variation  in  the  average  rainfall  of  the  different  dis- 
tricts. A  soil  in  Santa  Barbara  County  might  be  thoroughly  moist- 
ened at  the  beginning  of  the  growing  season,  owing  to  the  previous 
winter  rains,  to  a  depth  of  8  or  10  feet,  while  a  similar  soil  in  Riverside 
or  in  San  Diego  County  might  be  wet  to  a  depth  of  only  3  feet. 
Obviously,  the  irrigation  requirements  of  the  following  season  would 
be  quite  different  in  the  two  cases.  Table  III  shows  the  average  rain- 
fall in  several  of  the  sections  where  walnuts  are  grown. 

TABLE  III 
Showing  the  Normal  Rainfall  in  Several  Walnut  Growing  Sections 

Normal  annual 
Place  of  rainfall  in 

observation  inches 

Los  Angeles  14.6 

Napa    24.3 

Pomona*    19.2 

Riverside  10.6 

Santa  Barbara  17.1 

San  Diego 10.0 

San  Jose  16.8 

Santa  Ana  (Irvine)  13.3 

Santa  Rosa  32.0 

Stockton    14.6 

While  the  amount  of  seasonal  rainfall  may  be  taken  as  a  good  indi- 
cation of  the  amount  of  irrigation  water  necessary  to  supplement  it, 
it  should  be  borne  in  mind  that  other  factors,  such  as  frequency,  dura- 
tion, amount  of  individual  rains,  run-off,  and  season  of  occurrence, 
must  also  be  considered,  as  they  influence  the  proportion  of  the  total 
precipitation  which  penetrates  into  the  soil  and  is  available  to  the 
walnut  trees. 

The  water  requirements  are  higher  in  the  warm  inland  valleys,  with 
their  very  low  humidity  and  high  temperatures  during  the  growing 
season,  than  in  the  coastal  regions,  with  their  cooler  summers  and 
many  foggy  days. 


Average  of  8  years,  1913  to  1920  inclusive. 


152  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

It  may  be  said,  therefore,  that  the  walnut  grower  in  some  of  the 
southern  and  central  inland  districts  should  be  provided  with  a  water 
right  of  24  acre  inches  per  acre  per  year,  and  that  from  this  maximum 
the  requirements  diminish  until  in  some  individual  and  exceptional 
cases  in  Santa  Barbara,  Santa  Rosa,  and  Napa  counties  walnuts  may 
be  grown  successfully  without  any  irrigation. 

IRRIGATION  WATER  OF   GOOD   QUALITY  ESSENTIAL 

The  quality  of  the  irrigation  water  for  walnuts  is  as  essential  as 
the  quantity.  Relatively  small  amounts  of  the  "alkali"  salts  carried 
in  the  water  are  decidedly  harmful  to  walnut  trees.  From  many 
observations  made  during  the  past  five  years,  it  seems  that  the  walnut 
tree  is  the  most  sensitive  to  alkali  injury  of  all  the  orchard  trees  grown 
in  California.  The  minimum  amount  of  salts  which  will  prove  toxic 
to  walnut  trees  will  depend  upon  so  many  factors,  such  as  the  nature 
of  the  soil,  the  under-drainage,  the  rainfall,'  etc.,  that  it  may  be  im- 
possible to  state  the  exact  maximum  amount  of  salt  which  might  be 
applied  with  the  water  without  injury.  In  the  districts  of  heavy 
rainfall  and  in  well  under-drained  groves,  water  of  poor  quality  could 
no  doubt  be  used  longer  without  harm  to  the  trees  than  in  the  more 
arid  regions,  especially  where  hardpan  exists.  It  is  definitely  known 
that  walnut  groves  have  been  injured  severely  by  the  use  of  irrigation 
water  containing  only  340  parts  per  million  of  one  or  more  of  the 
so-called  "alkali"  salts.4 

The  particular  salts  which  the  water  contains  may  have  as  great 
a  bearing  on  the  injury  as  the  type  of  soil  and  other  natural  conditions 
of  the  orchard.  It  is  impracticable  to  include  a  prolonged  discussion 
of  the  effect  of  alkali  on  walnuts  in  this  publication ;  suffice  it  to  warn 
the  present  or  prospective  walnut  grower  of  the  importance  of  a  water 
supply  of  good  quality  before  continuing  or  establishing  a  walnut 
plantation.  It  is  especially  important  in  projecting  new  plantings 
to  have  the  prospective  water  supply  analyzed5  and  an  expert  opinion 
given  upon  its  suitability,  before  proceeding  to  develop  the  property. 

4  For  a  discussion  of  alkali  salts  in  irrigation  water,  see  Kelley  and  Thomas, 
' '  The  Effects  of  Alkali  on  Citrus  Trees, ' J  Univ.  of  Calif.  Experiment  Station  Bull. 
No.  318,  1920. 

5  The  University,  through  its  division  of  Plant  Nutrition  at  Berkeley  or  the 
chemical  division  of  the  Citrus  Experiment  Station  at  Riverside,  is  prepared  to 
analyze  irrigation  waters  free  of  charge,  and  give  an  opinion  on  their  suitability 
for  irrigation  purposes. 


Bulletin  332] 


WALNUT    CULTURE   IN    CALIFORNIA 


153 


VARIETIES 

The  original  walnut  plantings  in  California  were  "hard-shell"  and 
"paper-shell"  seedlings.  From  these  early  plantings,  the  next  gene- 
ration of  nuts  produced  a  nut  commonly  known  as  the  Santa  Barbara 
soft-shell.  The  vast  majority  of  the  seedling  walnut  groves  now  grow- 
ing in  California  consist  of  this  variety. 

From  these  latter  seedlings  many  superior  trees  have  been  noted 
during  the  past  15  or  20  years.  From  some  of  these,  through  propaga- 
tion by  grafting  and  budding,  (in  a  way  similar  to.  that  of  the  orig- 
ination   of   standard    varieties    of    fruits,    such    as    peaches,    apples, 


Fig.  2. — Effect  of  application  of  alkali  water.     Grove  on  left  used  water  of 
high  salt  content,  that  on  right,  good  water. 


oranges,  etc.),  new  varieties  of  walnuts  have  arisen.  The  varieties 
which  trace  back  to  the  class  of  seedlings  just  discussed  comprise  the 
vast  majority  of  the  sorts  now  planted  in  southern  California,  with 
the  exception  of  the  Eureka  variety,  a  brief  history  of  which  is  given 
under  the  description  of  this  variety.  Some  of  the  walnut  varieties 
commonly  planted  in  central  California,  such  as  the  Concord,  Fran- 
quette,  and  Mayette,  have  still  another  origin,  which  will  be  discussed 
under  the  heading  of  each  respective  variety. 

The  varieties  of  walnuts  chosen  for  planting  in  California  should 
be  those  most  marketable,  most  productive,  and  best  adapted  for 
growing  under  the  local  conditions  in  question.  At  present,  there  are 
comparatively  few  varieties  which  have  won  and  retained  their  popu- 
larity with  the  growers  and  the  marketing  trade.    Many  new  varieties 


154  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

have  been  originated  in  California-  during  the  past  twenty  years,  but 
most  of  them  have  been  propagated  only  in  a  very  limited  way,  and 
have  soon  fallen  into  disfavor,  owing  to  some  particular  shortcoming. 
New  varieties  of  the  future  should  be  a  decided  improvement  upon 
our  present  sorts;  otherwise  they  are  not  worthy  of  even  limited 
propagation. 

The  principal  economic  advantages  which  a  grafted  or  budded 
grove  of  the  best  varieties  possesses,  in  comparison  with  a  seedling 
walnut  grove,  are  (.a)  the  budded  nuts  sell  for  a  higher  price  than  the 
seedlings;6  (6)  a  grafted  grove  is  more  uniform  and  has  fewer  unpro- 
ductive trees  than  a  seedling  grove;  and  (c)  the  seedlings  are  more 
susceptible  to  blight  than  are  certain  varieties* which  are  budded  or 
grafted. 

There  probably  is  no  one  best  walnut  variety  for  planting  in  the 
whole  state  of  California.  With  the  varying  soil  and  climatic  condi- 
tions, the  success  or  failure  of  a  variety  may  depend  upon  the  question 
of  its  adaptation  to  the  specific  conditions.  In  reading  the  discussion 
of  the  varieties  in  the  following  pages  and  the  recommendations  which 
are  made  for  planting  in  the  several  localities,  it  should  be  borne  in 
mind  that  the  observations  concerning  the  varieties  have  been  very 
general.  In  many  instances  an  attempt  has  been  made  to  present  the 
consensus  of  the  opinions  of  several  observers,  such  as  walnut  growers, 
packing-house  experts,  and  nursery  men,  and,  although  the  varieties 
described  seem  to  be  the  best  for  commercial  planting  at  the  present 
time,  they  may  be  supplanted  in  the  future  by  varieties  entirely 
unknown  at  present,  or  by  varieties  which  may  not  be  highly  thought 
of  at  this  date.  The  selection  of  varieties  for  propagation  by  grafting 
and  budding  has  gone  on  for  so  short  a  period  (15  to  20  years)  that 
improvements  may  be  confidently  expected  in  the  future. 

In  considering  the  varieties  to  plant,  one  should  select  only  those 
which  meet  the  requirements  for  a  first-class  commercial  nut  to  be 
sold  in  the  so-called  "budded"  grade.7 


6  The  average  price  for  seedling  walnuts  sold  by  the  California  Walnut  Growers ' 
Association  during  the  12-year  period,  1909  to  1920,  inclusive,  is,  for  No.  1  soft- 
shell  nuts,  18.2  cents  per  pound.  During  the  same  period,  Fancy  Budded  nuts 
averaged  21.3  cents  per  pound. 

7  At  present  only  a  portion  of  the  varieties  of  the  California  walnuts  are  sold 
at  wholesale  under  their  varietal  names.  The  Eureka,  Franquette,  Mayette,  Payne, 
and  Concord  are  sold  at  such;  the  Placentia,  which  comprises  a  greater  bulk  than 
all  other  grafted  varieties  combined,  is  sold  under  the  general  term  "budded." 

As  the  ' l  budded ' '  nuts  are  graded  in  the  packing  houses  of  the  California 
Walnut  Growers'  Association,  they  pass  through  a  large  galvanized  iron  cylindrical 
grader,  10  feet  long  and  40  inches  in  diameter.  This  grader,  which  revolves 
slowly,  at  a  6-inch  pitch,  has  a  capacity  for  propely  grading  about  one  and  a  half 
tons  of  nuts  per  hour  by  passing  them  over  somewhat  less  than  8000   l%G-ineh 


Bulletin  332]  WALNUT  CULTURE  IN   CALIFORNIA  155 

A  heavy  shell  and  a  firmly  sealed  nut  are  essential  to  withstand 
the  handling  in  the  grading,  packing,  and  commercial  operations 
without  cracking  open.  A  nut  which  is  partly  cracked  open  soon 
becomes  rancid  and  must,  therefore  be  eliminated  from  first-class 
grades.  The  proportion  of  kernel  to  the  total  weight  in  a  sample  of 
carefully  graded  nuts  is  slightly  less  than  50  per  cent. 

DESCRIPTION    OF    VARIETIES8 

(Described  in  order  of  their  popularity  in  1920.) 

PLACENTIA 

The  Placentia  may  at  present  be  considered  the  best  varietj^  of 
walnut  which  has  been  proved  thoroughly  successful,  commercially, 
in  southern  California.  At  present  most  of  the  nuts  sold  in  the 
"budded"  grade  are  of  this  variety.  The  defects  of  this  variety  will 
be  stated  first,  and  its  good  qualities  later. 

Probably  the  greatest  defect  of  Placentia  is'  its  pronounced  sus- 
ceptibility to  blight.  During  1915  and  1916  many  of  the  Placentia 
groves  blighted  as  badly  as  the  average  seedling  groves,  although  from 
1917  to  1920,  when  there  was  little  blight,  this  variety  was  particularly 
profitable.  The  second  defect  is  the  tendency  of  the  nuts -to  spring 
open  at  the  apex  if  they  are  dried  too  rapidly  by  exposure  in  the  sun 
during  the  curing  process. 

The  variety  is  readily  propagated  by  grafting  upon  the  black 
walnut,  making  a  thrifty,  rapidly  growing  tree.  The  Placentia  usually 
bears  young  in  southern  California,  and  produces  very  good  crops. 
It  is  regular  in  its  bearing  habits  and  lacks  the  tendency  of  some  sorts 
to  bear  every  other  year.  The  nut  has  the  desirable  size  for  com- 
mercial sale,  being  oval,  with  a  fairly  smooth  shell.  The  nuts  tend 
to  vary  in  form,  and  some  strains  are  more  nearly  round  and  some- 
what roughened.  The  nuts  are  not  well  sealed.  The  shell  is  thin, 
but  strong.  The  kernel  is  smooth,  plump,  and  light  colored,  and  is 
taken  as  a  standard  of  quality  and  appearance  for  the  budded  grade 
of  nuts. 


square  opening's.  The  nuts  which  drop  through  the  openings  are  known  as  the 
Standard  Budded  grade;  those  which  are  too  large  to  pass  through  the  openings 
come  out  at  the  lower  end  of  the  grader  and  are  known  as  the  Fancy  Budded 
grade.  The  latter  grade,  composed  of  the  larger  nuts,  constitutes  the  highest 
priced,  so-called  " budded"  nuts.  In  addition  to  the  size  requirements  here  de- 
scribed, an  ideal  budded  nut  should  have  a  plump,  sound  kernel  of  a  bright,  light- 
straw  color,  and  be  free  from  mould.  The  shell  must  be  of  a  clear,  bright  color, 
free  from  any  discoloration. 

*  For  a  more  complete  description  and  history  of  the  following  varieties,  as 
well  as  the  more  uncommon  ones,  see  Univ.  of  Calif.  Experiment  Station  Bull. 
No.  231,  1912,  and  U.  S.  Dept.  of  Agr.,  Bur.  of  Plant  Indus.,  Bull.  No.  254,  1913. 


156  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

The  Placentia  variety  has  been  grown  with  marked  success 
throughout  the  coastal  sections  of  southern  California,  and  some  of 
the  best  grafted  trees  in  the  inland  valleys  of  the  south  are  of  this 
variety.  The  Placentia  has  not  become  popular  with  the  walnut 
growers  of  central  or  northern  California,  because  it  seems  to  lose 
there  some  of  its  tendency  for  precocity  and  heavy  production,  and 
is  further  hampered  by  its  production  of  dark-meated  nuts. 


EUEEKA9 

This  variety  was  originally  selected  because  of  its  supposed  resist- 
ance to  walnut  blight,  the  heavy  production  of  the  parent  tree,  and 
the  exceedingly  high  quality  of  the  nut.  The  supposed  resistance  to 
blight  is  largely  mythical.10  The  mistake  was  due  probably  to  the 
great  variation  in  blight  occurrence  during  different  seasons,  with  the 
possibility  that,  during  the  early  years  of  observation  of  this  variety, 
the  blight  may  not,  have  been  normal  in  its  prevalence  or  season  of 
occurrence  in  the  district  surrounding  the  original  tree. 

The  Eureka  is  not  usually  so  readily  propagated  upon  the  black 
walnut  root  as  some  of  the  Santa  Barbara  soft-shell  varieties.  The 
trees  are  vigorous  growers,  although  the  growth  may  not  be  so  great 
as  that  of  the  Placentias  in  the  same  orchard.  The  Eureka  is  later 
in  reaching  an  age  of  profitable  bearing  than  the  Santa  Barbara  soft- 
shell  varieties  in  southern  California,  and  although  the  high  quality 
of  the  Eureka  nuts  causes  them  to  sell  for  more  than  the  Placentia 
the  difference  averages  only  about  2y2  cents  a  pound.  This  higher 
price  has  not  made  up  for  the  lighter  crops  which  are  obtained,  in 
most  instances,  during  the -first  10  to  15  years.  Comparisons  cannot 
be  made  between  mature  trees  of  these  varieties,  as  the  Eureka  has 
been  so  recently  introduced.  In  some  instances  the  Eureka  seemed 
to  have  a  tendency  toward  biennial  bearing.  It  is  to  be  hoped  that 
this  variety  may  prove  more  productive  in  southern  California  as 
the  trees  become  15  to  20  years  old,  as  the  nut  itself  has  many  points 
to  commend  it,  and  large  acreages  have  been  planted  with  this  variety. 

The  tree  blooms  much  later  than  most  other  varieties  and  often 
escapes  injury  to  the  flowers  bj^.  late  spring  frosts.    Its  harvest  season 


9  The  parent  tree  of  the  original  Eureka  tree  was  a  Persian  or  Kaghazi  type 
of  walnut  which  grew  on  the  old  Meek  estate  near  Hayward,  California.  The 
seedling  which  developed  into  the  original  tree  is  now  a  mature  tree  near  Fullerton, 
California. 

io  For  account  of  the  blight  occurrence  in  the  Eureka  walnut,  see  Fawcett  and 
Batchelor,  '  f  An  Attempt  to  Control  Walnut  Blight, ' '  Monthly  Bulletin,  Dept.  of 
Agric,  State  of  California,  May-June,  1920,  p.  177. 


Bulletin  332] 


WALNUT    CULTURE   IN    CALIFORNIA 


15' 


is  fully  three  weeks  later  than  that  of  seedling  nuts,  or  of  the  Placen- 
tia.  This  is  a  distinct  disadvantage  in  marketing  the  crop  in  the 
eastern  cities  in  time  for  the  holiday  trade. 


Fig.  3. — Placentia. 

The  Eureka  variety  has  suffered  a  great  deal  from  injury  caused 
by  being  frosted  during  the  early  fall.  Whether  this  is  due  to  a 
characteristic  of  late  ripening,  or  to  the  possibility  that  the  Eureka 
has  been  planted  in  places  more  subject  to  frost  than  other  varieties, 
cannot  be  definitely  stated  at  this  time. 


158  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

The  Eureka  may  escape  with  less  injury  from  sunburn  than  do 
some  varieties,  as  its  leaves  are  larger  and  the  nuts  are  borne  in  a 
position  on  the  twigs  which  affords  more  natural  protection  than  most 
varieties  have.  This  does  not  make  the  Eureka  entirely  immune  from 
sunburn  injury,  however,  especially  if  the  hot  weather  occurs  in  June 
or  early  July,  when  the  nuts  are  only  half-grown,  and  before  the 
foliage  is  fully  developed. 

The  Eureka  nut  is  easily  distinguished  from  any  other  variety 
grown  in  southern  California  by  its  pronounced  elongation,  rather 
straight  parallel  sides,  slightly  rounding  to  square  ends,  with  the  apex 
usually  broader  and  more  nearly  square  than  the  base.  The  color  of 
its  shell  is  a  bright  straw,  of  a  lighter,  more  attractive  shade  than  the 
typical  soft-shell  nuts.  The  extra  heavy  shell  and  the  exceptionally 
and  completely  firm  "seal"  of  the  nut,  make  it  a  model  nut  for  good 
keeping  qualities,  and  for  withstanding  commercial  handling.  A 
well-grown  Eureka  nut  is  filled  to  its  full  capacity  with  a  light,  cream- 
colored,  plump,. waxy,  kernel,  with  rather  deep  convolutions,  possess- 
ing the  very  best  of  eating  qualities.  The  Eureka  is  richer  and  freer 
from  astringency  than  the  soft-shell  varieties. 

This  variety  has  made  many  friends  in  central  California  around 
Stockton  and  San  Jose,  and  in  some  of  the  hot  inland  valleys  of 
southern  California,  where  it  seems  to  be  more  promising  for  profitable 
production  than  the  Placentia. 

The  kernel  of  the  Eureka  walnut  will  not  fill  out  perfectly  on  all 
four  quarters  unless  the  trees  are  amply  supplied  with  soil  moisture. 
The  variety  has  been  disappointing  in  some  of  the  central  California 
districts,  where  the  trees  have  not  been  adequately  supplied  with 
irrigation  water. 

On  the  merits  of  its  eating  quality  and  the  attractive  appearance 
of  its  kernel  it  might  be  expected  to  out-sell  the  Placentia  more  than 
2y2  cents  a  pound ;  however,  this  average  figure  of  the  past  is  the  only 
safe  guide  in  projecting  a  business  undertaking.  It  is  to  be  hoped 
that  this  superior  nut  will  eventually  prove  to  have  the  tree  and 
bearing  characteristics  necessary  to  make  it  the  leading  variety  in 
southern,  and  more  especially,  central  California ;  the  evidence  at  hand, 
however,  does  not  justify  an  unqualified  business  confidence  that  such 
hopes  will  be  realized. 


BULLETIN   332]  WALNUT    CULTURE   IN    CALIFORNIA  159 

EHRHAEDT 

This  variety  has  only  recently  been  planted  commercially.11 

Origin. — The  location  of  the  original  seedling  tree  from  which  this 
variety  sprang  is  apparently  unknown.  The  tree  was  first  selected 
and  propagated  by  the  late  D.  C.  Disher,  who  sold  fifty  grafted  trees 
to  V.  E.  Ehrhardt,  of  Santa  Ana,  during  the  spring  of  1906.  The 
variety  had  not  been  given  a  name  by  Mr.  Disher  at  that  time,  but 
the  above  trees  were  delivered  as  a  substitute  for  Placentia,  with 
Mr.  Ehrhardt 's  sanction.  Mr.  Disher 's  only  explanation  and  descrip- 
tion of  the  trees  was  that  they  were  propagated  from  a  selected  Santa 
Barbara  soft-shell  seedling  tree  of  unusual  superiority,  and  he  further 
announced  his  intention  of  propagating  from  this  tree  extensively 
and  offering  it  to  the  trade  as  a  new  variety.  Unfortunately,  Mr. 
Disher  died  soon  after  the  above  transaction,  without  disclosing  the 
location  of  the  original  tree. 

The  writer's  attention  was  first  called  to  these  grafted  trees  by 
Mr.  Charles  Knowlton,  of  Fullerton,  in  the  spring  of  1915.  Since 
then,  many  observations  have  been  made,  especially  to  determine  the 
amount  of  blight  prevalent  and  the  yields  of  the  several  seasons.  The 
variety  has  attracted  considerable  attention  locally,  and  is  now  com- 
monly known  as  the  "Ehrhardt." 

Following  is  a  description  of  the  nut  and  the  trees : 

Nut 

Size. — Larger  than  the  average  Santa  Barbara  soft-shell  nut, 
requiring  approximately  34-40  nuts  to  weigh  a  pound. 

Form. — Broadly  oval,  base  rounding,  sometimes  slightly  pointed, 
apex  rather  blunt,  suture  shallow,  flange  rather  prominent. 

Surface. — Medium  to  smooth. 

Color. — Medium  light  brown. 

Cracking  Quality. — Nuts  well  sealed  at  both  ends"  when  properly 
handled  at  harvest  season ;  kernels  readily  cracked  out  whole. 

Kernel. — Very  plump  and  well  developed,  usually  of  a  light  tan 
color.  Good  market  type,  averaging  about  50  per  cent  of  the  total 
weight  of  the  nuts. 

Flavor. — Mild,  pleasant,  and  free  from  any  decided  astringent 
character. 


ii  For  a  detailed  description  and  history  of  all  other  varieties  mentioned  here, 
the  reader  has  been  referred  to  publications  which  were  issued  before  the  public 
appearance  of  the  Ehrhardt.  A  similar  account  of  this  walnut  is  therefore  given 
here. 


160  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

Tree 
Foliation  Period. — Quite  early,  about  same  as  Placentia. 
Growth. — Vigorous,  typical  of  this  type  of  nut-tree,  producing  a 
large  amount  of  lateral  twigs. 
Foliage. — Dense. 

Harvest  Season. — Medium  to  early. 

Precocity. — Very  early  producer;  apparently  this  is  second  to 
none.  There  are  38  of  the  original  planting  on  Mr.  Ehrhardt's  prop- 
erty at  the  present  time ;  their  average  yields  for  six  years  are  shown 
in  the  table  under  productiveness. 

Productiveness. — This  is  very  heavy  producing  variety,  as  shown 
by  the  following  yields  of  the  original  38  trees  on  Mr.  Ehrhardt's 
property. 

10th  season  (1915)  80  lbs.  per  tree. 
11th  season  (1916)  76  lbs.  per  tree. 
12th  season  (1917)  79  lbs.  per  tree. 
13th  season  (1918)  81  lbs.  per  tree. 
14th  season  (1919)  117  lbs.  per  tree. 
15th  season   (1920)     98  lbs.  per  tree. 

The  Ehrhardt  walnut  is  of  the  Santa  Barbara  soft-shell  type, 
closely  resembling  in  some  ways  the  Placentia.  It  is  somewhat 
rougher,  slightly  larger  and  better  "sealed."  The  kernel  gives  the 
impression  of  being  plumper  than  that  of  the  Placentia,  because  its 
convolutions  are  very  shallow. 

It  is  as  yet  too  soon  to  make  definite  statements  concerning  the 
commercial  value  of  this  variety.  It  will  probably  succeed  where  the 
Santa  Barbara  soft-shell  succeeds.  After  observing  the  thirty-eight 
trees  on  Mr.  Ehrhardt's  place  for  six  years,  it  seems  reasonable,  to 
expect  that  the  Ehrhardt  will  some  day  prove  its  superiority  to  the 
Placentia  as  a  heavy  yielding  variety.  The  Ehrhardt  trees  were  nearly 
free  from  blight  during  1915  and  1916,  when  this  disease  was  very 
widespread.  At  no  time  did  the  writer  observe  as  much  as  5  per  cent 
of  blighted  nuts  on  the  Ehrhardt  trees,  while  Placentia  trees  in  the 
same  orchard  were  blighted  as  badly  as  45  per  cent,  and  seedlings 
showed  65  per  cent  by  actual  count.  Since  1916,  the  outbreaks  of 
blight  have  not  been  severe  enough  to  really  test  this  character  of 
the  Ehrhardt.12    However,  whether  largely  resistant  to  blight  or  not, 


12  No  explanation  can  be  offered  of  the  freedom  from  blight  of  the  Ehrhardt 
trees  during  1915  and  1916.  The  Placentia  trees  were  of  the  same  age  or  younger, 
growing  in  the  same  orchard,  and  across  a  narrow  highway.  The  two  varieties 
bloom  at  practically  the  same  time. 


Bulletin  332] 


WALNUT   CULTURE   IN    CALIFORNIA 


161 


its  tendency  to  produce  very  heavy  crops  of  high-grade  nuts,  appar- 
ently out-yielding  the  Placentia,  makes  this  new  variety  worthy  of 
commercial  trial. 


Fig.   4. — Eureka. 


The  Ehrhardt  is  readily  propagated  upon  the  black  walnut  root, 
making  a  tree  of  medium  vigor  which  bears  at  an  early  age.  The 
nuts  are  considered  very  promising  by  the  wholesale  trade.  They 
easily  qualify  in  the  budded  grade  of  walnuts. 


162  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

PAYNE 

Although  the  parent  tree  of  this  variety  was  discovered  by  G.  P. 
Payne,  near  Campbell,  California,  in  1898,  the  variety  has  not  been 
widely  planted  and  only  in  recent  years  has  it  been  propagated  in 
any  great  quantity.  The  precocity  of  this  variety  for  heavy  pro- 
duction is  probably  the  chief  characteristic  which  has  brought  it  into 
prominence  during  the  past  four  years.  It  must  be  kept  in  mind, 
however,  that  walnut  blight  has  not  been  prevalent  to  a  great  degree 
during  1918  to  1920.  Moreover,  the  Payne  is  notoriously  subject 
to  blight  during  seasons  of  bad  outbreaks.  The  recent  rise  in  popu- 
larity of  this  variety  may  be  very  much  altered  if  walnut  blight 
prevails  in  the  future  as  it  did  during  the  seasons  of  1915  and  1916. 

The  Payne  tree  makes  a  rather  slow  growth,  possibly  because  of 
its  very  heavy  production  as  a  young  tree.  The  nuts  are  borne  rather 
prominently  on  the  outside  of  the  trees  and  are  thus  subject  to  severe 
sunburning  in  the  inland  valleys. 

The  nut  is  oblong,  rather  pointed  at  the  apex;  shell  of  medium 
thickness,  somewhat  pitted;  well  sealed;  kernel  full,  with  moderate 
convolutions  and  of  good  quality.  With  the  heavy  production  of  the 
oldest  trees  growing  at  present,  there  seems  to  be  a  reduction  in  the 
size  of  the  nut,  making  a  high  percentage  of  No.  2  grade.  This  nut 
has  not  been  sold  in  the  Fancy  Budded  grade,  but  has  been  sold  under 
its  varietal  name.  During  the  1921  season  it  sold  for  less  than  the 
California  Walnut  Growers'  Association  "Fancy  Budded"  grades, 
but  for  one  cent  per  pound  more  than  the  seedlings. 

The  greatest  popularity  of  this  variety  is  found  perhaps  in  the 
Stockton,  San  Jose,  San  Fernando,  and  Elsinore  districts.  In  the  first 
mentioned  district  it  is  as  popular  as  the  Eureka.  It  might  be  suit- 
able to  interplant  in  a  Eureka  grove.  Thus,  the  first  ten  years  of  the 
slow-developing  productivity  of  the  Eureka  would  be  bridged  over 
by  this  prolific  and  precocious  variety.  In  the  districts  of  the  coastal 
counties  where  the  blight  is  most  regularly  severe  and  prevalent,  the 
Payne  variety  certainly  cannot  be  recommended  for  permanent  trees. 
The  greatest  value  of  this  variety  may  prove  to  be  its  precocity  and 
thus  its  value  as  a  temporary  interplanted  tree  among  slower  develop- 
ing but  superior  varieties. 

CONCOED 

The  Concord  is  one  of  the  best  tested  varieties  for  the  central 
portions  of  California,  especially  in  Contra  Costa  and  Napa  counties, 
where  many  thousand  trees  are  growing.     In  these  districts,  it  has 


Bulletin  332] 


WALNUT    CULTURE   IN    CALIFORNIA 


163 


proved  to  be  a  vigorous,  thrifty  tree,  and  an  annual  producer  of 
medium  sized  crops.  The  Concord  blooms  nearly  as  late  as  the 
Eureka,  but  possibly  the  nuts  mature  a  week  earlier  than  the  latter 


Fig.  5.— Ehrhardt. 

variety.  The  trees  are  fairly  precocious.  The  nuts  are  elongated  and 
somewhat  pointed  at  the  apex ;  the  shell  is  rather  smooth ;  the  nuts 
are  poorly  sealed,  the  kernel  fairly  plump,  with  medium  to  deep 
convolutions,  only  medium  light-colored  and  of  good  quality.  This 
variety  is  not  so  well  received  by  the  wholesale  trade  as  the  Placentia 


164  UNIVERSITY    OF    CALIFORNIA — EXPERIMENT    STATION 

or  Eureka,  It  is  doubtful  if  it  should  be  planted  where  the  two  above- 
mentioned  varieties  will  succeed.  In  southern  California  the  Concord 
does  not  yield  so  well  as  the  soft-shell  varieties,  so  far  as  they  have 
been  observed  by  the  writer.  Possibly  the  oldest  Concord  trees  in 
southern  California  are  a  group  of  top-grafted  trees  on  the  Pasadena 
sewer  farm.  The  trunks  are  soft-shell  seedlings,  while  the  tops,  now 
10  or  12  years  old,  are  as  large  as  15  to  20  year  old  trees.  This  group 
of  Concords  has  produced,  annually,  a  medium  crop  during  the  past 
four  years.  Nothing  definite  can  be  said  about  the  blight,  as  it  has 
not  been  severe  anywhere  in  this  orchard  during  the  period  under 
consideration. 

FEANQUETTE 

This  variety  has  been  tested  longer  in  central  California  than  any 
other.  Successful  commercial  plantings  of  Franquettes  occur  in 
Sonoma,  Napa,  Contra  Costa,  Santa  Clara,  and  Tulare  counties.  This 
variety  has  many  adherents  in  these  sections  because  of  its  regular 
bearing  of  moderate  size  crops  of  excellent  quality.  The  Franquette 
has  borne  too  lightly  in  the  Stockton  district  to  be  recommended  for 
commercial  use.  In  southern  California,  the  Franquette  has  not 
proved  commercially  successful,  owing  to  its  slow  growth  and  light 
crops. 

The  Franquette  is  late  in  reaching  a  bearing  age.  It  blooms  very 
late,  and  the  harvest  season  is  too  late  for  the  most  advantageous 
disposal  of  the  crop  for  the  eastern  holiday  trade.  Owing  to  its  late 
blooming  it  usually  escapes  injury  by  late  spring  frosts. 

The  nut  is  large,  elongated,  pointed,  fairly  smooth,  and  of  a  lighter, 
clearer  and  more  attractive  color  than  the  soft-shell  types.  The  shell 
is  thin,  but  well  sealed,  and  fairly  well  filled  with  a  very  light  colored, 
moderately  plump  kernel.  The  eating  qualities  of  the  Franquette  are 
unsurpassed  by  any  variety  grown  in  California.  The  high  quality 
of  this  nut  makes  it  sell  for  a  higher  price  than  most  other  varieties 
grown  in  the  state,  except  the  Eureka, 

Where  ample  irrigation  water  is  available,  the  Eureka  seems  to  be 
gaining  in  favor  more  rapidly  than  the  Franquette.  In  sections  of 
rather  heavy  winter  rainfall,  where  walnuts  are  usually  grown  by 
dry-farm  methods,  it  is  probable  that  the  Franquette  will  be  chosen, 
as  the  Eureka  may  not  produce  a  well-filled,  plump  kernel  under  such 
conditions.  The  heavy  weight  of  the  Eureka  nuts  may  make  them 
preferable  even  in  the  localities  where  the  Franquette  is  successfully 
grown,  if  the  soil  moisture  conditions  are  favorable  to  the  former 
variety. 


Bulletin   332]  WALNUT   CULTURE   IN    CALIFORNIA  165 

ROOTSTOCKS13    AND    CHOICE    NURSERY    TREES 

CHOICE   OF   EOOTSTOCK 

The  choice  of  rootstocks  for  the  walnut  has  narrowed  down  during 
the  past  20  years  to  a  predominating'  preference  for  the  northern  Cali- 
fornia black  walnut  (J.  hindsii).  The  seed  is  shipped  commercially 
by  several  agencies  from  central  California  for  this  purpose.  Groups 
of  dooryard  and  border  trees  usually  furnish  this  seed.  Some 
nursery  men  in  southern  California  have  located  and  use  seed  from 
particularly  vigorous  black  walnuts  of  this  species,  which  have  been 
planted  locally.  The  seedlings  from  a  group  of  such  trees  vary  con- 
siderably, and  in  some  instances  certain  trees  are  especially  valued 
because  of  their  tendency  to  produce  uniform  and  vigorous  seed- 
lings. This  species  of  black  walnut  will  apparently  withstand  a  more 
unfavorable  soil  condition  than  the  seedling  English  walnut.  Its 
resistance  to  oak-root  fungus  (ArmMaria  mellea)  is  also  a  valuable 
characteristic  of  this  rootstock.  The  northern  California  black  walnut 
is  more  vigorous  as  a  nursery  tree  than  the  eastern  black  walnut, 
which  was  formerly  used  to  some  extent,  only  to  be  discarded  in  recent 
years. 

The  southern  California  black  walnut  (J.  Galifornica)  is  seldom 
used  at  present,  because  of  its  tendency  to  sucker  profusely  at  the 
crown  as  an  orchard  tree,  and  its  susceptibility  to  root-rot  troubles 
on  heavy  or  poorly  drained  soils. 

The  paradox-hybrid  walnut,  which  is  a  cross  between  the  English 
and  any  of  the  black  walnuts,  makes  a  remarkably  rapid  growing  tree. 
Mature  trees  may  have  a  spread  of  over  sixty  feet.  The  vigor  of  this 
first  generation  hybrid  has  been  mainly  responsible  for  its  popularity 
among  some  planters.  Such  trees  cannot  be  secured  in  quantity,  how- 
ever, so  that  their  use  will  always  be  restricted.  The  increased  vigor 
of  the  first-generation  hybrid  is  not  carried  on  to  following  genera- 
tions; in  fact,  the  first-generation  hybrid  trees  are  usually  nearly 
barren  and  their  seedlings  show  many  types  of  growth  and  vigor, 
making  undesirable  rootstocks. 

The  Koyal-hybrid  walnut  is  a  cross  between  the  eastern  black 
walnut  and  the  California  black  walnut.  It  is  thought  by  some,  that 
the  second-generation  Royal-hybrid  is  equal  or  even  superior  to  the 
northern  black  walnut  as  a  rootstock.  There  may  be  such  hybrid 
strains  which  justify  this  confidence,  but,  as  a  whole,  it  cannot  be  said 
that  such  superiority  has  been  proved.  Seedlings  from  some  of  the 
Royal-hybrid  trees  produce  uniformly  vigorous  trees,  while  those  from 

13  For  details  of  nursery  propagation,  see  Univ.  of  Calif.  Experiment  Station 
Bull.  No.  231,  1912. 


166  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

others  are  variable  to  a  great  degree  and  lack  vigor.    Very  likely  the 
pollinating  parent  has  much  to  do  with  this  lack  of  consistency. 

METHOD  OF  PEOPAGATION 
The  English  walnut  is  usually  grafted  on  to  the  black  walnut  root 
during  the  early  spring,  using  a  whip  graft.     It  can  be  budded,14 
however,  and  some  nurserymen  prefer  this  method. 

EEQUIEEMENTS  FOE  GOOD  NUESEEY  TEEES 
In  the  case  of  the  grafted  trees,  the  black  walnut  trees  are  grown 
one  year  in  the  nursery  and  then  grafted  in  late  winter ;  the  grafted 
trees  should  grow  from  6  to  12  feet  the  summer  following.  Such  trees 
are  trained  to  a  whip  growth  and  usually  sell  according  to  size,  a 
premium  being  placed  upon  the  larger  trees.  The  pre-war  price  of 
well-grown  walnut  trees  was  from  $1.00  to  $1.50 ;  during  1919-21 
trees  have  sold  as  high  as  $3.00  each  for  6  to  8-foot  trees.  A  medium 
sized  tree  (8  to  10  feet)  may  be  preferable  to  either  an  extremely  large, 
or  a  small  tree.  A  very  large  tree  may  be  injured  considerably  in 
digging  from  the  nursery,  while  a  small  tree  may  be  a  stunted  tree 
with  a  poor  root  system  and  never  make  a  first-class  orchard  tree. 

STARTING  THE  YOUNG  ORCHARD 

LAYING  OUT  THE  OECHAED 

From  12  to  27  walnut  trees  per  acre  are  found  in  the  mature 
orchards.  General  observation  and  the  opinions  of  many  walnut 
growers  seem  to  agree  that  the  plantings  of  the  past  have  been  made 
with  the  trees  too  close  together  in  many  instances. 

If  the  trees  are  spaced  60  feet  apart  each  way,  planted  on  the 
square  system,  it  will  require  12  trees  per  acre ;  orchards  planted  in 
this  manner  are  among  the  most  productive  in  the  state.  When 
spaced  60  feet  apart,  the  individual  trees  have  ample  room  to  develop 
fully  and  a  large  proportion  of  the  nuts  are  produced  on  the  side 
branches.  With  only  12  trees  to  the  acre  the  root  systems  of  the 
trees  are  not  so  crowded  as  in  closer  planting,  and  the  trees  maintain 
a  healthy  vigorous  growth  of  new  fruiting  wood  when  they  are  thirty 
years  old,  or  older. 

i*  The  question  is  often  asked:  "What  is  a  budded  walnut?"  The  grade  of 
nuts  sold  under  the  name  '  *  budded ' '  has  no  doubt  prompted  this  question  in  many 
instances.  A  budded  nut  is  any  one  of  the  varieties  which  will  grade  according 
to  the  standard  of  perfection  set  by  the  Placentia.  The  tree  which  bears  such 
a  nut  has  been  produced  by  budding  or  grafting  a  bud  or  scion  of  the  variety  in 
question  on  a  black  walnut  rootstock.  The  English,  or  the  hard-shell,  are  seldom 
used  as  a  rootstock  at  present.  If  budded,  the  work  is  usually  done  in  August, 
using  a  patch  or  a  shield  bud  from  the  current  season's  growth. 


Bulletin   332]  WALNUT   CULTURE  IN    CALIFORNIA  167 

Jn  the  close  plantings,  where  the  trees  are  only  40  to  50  feet  apart, 
the  side  branches  are  shaded  most  of  the  day,  the  fruit  spurs  on  the 
lower  branches  soon  die,  and  the  crop  is  borne  mainly  in  the  tops  of 
the  trees.  A  closely  planted  grove  is  illustrated  by  Fig.  6.  Such 
orchards  have  not  maintained  their  productivity  so  well  as  those 
planted  further  apart.  Plantings  where  the  trees  are  now  growing 
40  or  45  feet  apart  can  be  thinned  out  by  removing  every  other  tree 
in  each  row  so  that  the  trees  alternate,  or  stand  opposite  a  space  in 
the  adjacent  rows,  then  the  remaining  trees  will  stand  approximately 
57  or  63  feet  apart  respectively,  in  the  rows,  running  diagonally  across 
the  orchard.     This  may  be  made  clearer  by  the  following  diagram : 


DIAGEAM  I 

Showing  Method  of  Thinning  Out  Too  Closely  Planted  Orchards 


O  Permanent  trees  to  remain. 
X  Trees  to  be  pulled  out. 

Figure  7  shows  a  view  in  an  old  seedling  orchard  after  the  thin- 
ning process  had  advanced  to  the  stage  of  cutting  off  the  tops  of  the 
condemned  trees,  ready  for  the  stump  puller.  This  paragraph  con- 
cerning thinning  old  orchards  is  placed  here  under  the  general  heading 
of  planting  with  the  realization  that  many  prospective  walnut  planters 
can  benefit  by  the  experience  of  those  who  have  blazed  the  way  in 
this  industry. 

One  of  the  most  favored  systems  of  planting  at  present  is  to  plant 
the  trees  in  rows  60  feet  apart  with  a  distance  of  30  feet  between  the 
trees  in  the  rows.  The  trees  may  be  all  of  one  variety,  or,  in  localities 
where  the  varietal  adaptation  is  in  question,  two  varieties  may  be 
planted  alternately.  When  planted  according  to  this  latter  method, 
there  is  less  hazard  of  planting  the  wrong  sort,  as  one  may  have  a 
choice  between  two  varieties  after  they  reach  a  bearing  age.     Such 


168  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

plantings  are  usually  thinned  out  when  the  trees  are  from  10  to  14 
years  old,  according  to  the  variety,  soil,  water  and  climatic  conditions. 
They  should  not  be  left  until  they  crowd  badly.  By  this  method, 
properly  carried  out,  nearly  twice  the  tonnage  per  acre  may  be  ex- 
pected during  the  first  10  or  14  y ears'  growth  as  where  only  the 
permanent  trees  are  planted.  At  the  same  time,  there  is  practically 
as  much  room  for  intercrops,  such  as  beans,  during  the  early  life  of 
the  orchard  as  where  only  12  trees  are  planted  per  acre.  The  extra 
expense  of  buying  and  planting  filler  trees  may  be  paid  for  by  part 
of  a  year's  crop,  while  all  other  extra  expense  due  to  the  filler  trees, 
such  as  pruning,  extra  water,  etc.,  is  of  minor  importance. 

The  following  diagram  illustrates  this  plan  of  setting  out  filler 
trees. 


OXOXOXOXO 


oxoxoxoxo 

o.-  Permanent   trees 
x.-  Filler  trees 


--:::  space   for   intercrops 

CAEE  OF  TEEES  BEFOEE  PLANTING 
If  the  trees  are  received  from-  the  nursery  before  the  ground  is 
ready  for  planting,  they  should  be  unpacked  and  heeled  in,  in  a  shady 
place.  Water  the  soil  around  the  roots  thoroughly  after  heeling  them 
in.  If  it  is  more  practical,  the  trees  may  be  held  for  a  time  under  a 
shed  with  the  roots  packed  in  damp  sawdust  or  shavings. 

Just  before  planting,  the  root  system  should  be  trimmed,  to  cut 
off  any  mutilated  portions  of  the  tap  root  or  lateral  roots.  In  fact,  if 
the  ends  are  all  cut  back  slightly  with  a  sharp  pruning  knife,  leaving 
a  clean  smooth  cut,  the  new  rootlets  will  start  out  much  more  readily 
from  such  a  surface  than  from  a  ragged  cut  or  a  torn  and  bruised  root. 


Bulletin  332] 


WALNUT    CULTURE   IN    CALIFORNIA 


169 


^^^M^^^m^^^^^^ 


Fig.  6. — The  trees  iu  this  orchard  were  planted  too  close,  being  spaced  40  feet. 
They  are  producing  walnuts  only  in  the  tops  and  will  soon  have  to  be  thinned  in 
order  to  obtain  a  satisfactory  yield  per  acre. 


Fig.  7. — A  grove  showing  every  other  tree  removed  in  each  row,  so  that  the 
trees  alternate. 


170  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

PLANTING  THE  YOUNG  OECHAED 

Walnut  trees  should  be  planted  during  January  and  February 
so  that  the  soil  may  thoroughly  settle  around  the  roots,  and  growth 
start  with  the  beginning  of  the  normal  growing  season,  which  is  usually 
early  March,  in  southern  California.  Holes  should  be  dug  deep 
enough  to  allow  room  for  the  full  length  of  the  tap  root,  which  may 
be  from  18  to  30  inches.  The  lateral  roots  may  be  6  or  8  inches  long, 
and  the  hole  should  be  wide  enough  to  accommodate  them  without 
cutting  back  except  to  remove  mutilated  portions. 

In  filling  in  the  soil  around  the  roots,  it  is  advisable  to  use  the  top 
soil,  tamping  it  thoroughly  without  bruising  the  roots.  A  better  stand 
of  trees  may  be  expected,  and  a  more  prompt  growth  in  the  spring, 
if  the  young  trees  are  thoroughly  irrigated  as  soon  as  they  are  planted. 
This  can  be  accomplished  by  running  a  single  furrow  along  the  row 
and  cutting  the  water  in  at  each  basin  left  around  the  trees  as  the 
holes  are  filled.  Irrigation  at  this  time  may  not  be  essential  from  the 
point  of  view  of  soil  moisture  available,  but  it  will  be  worth  the  pains 
as  a  means  of  thoroughly  settling  the  soil  around  the  roots  to  prevent 
them  drying  out.  A  few  light  rains  should  not  mislead  the  planter 
and  cause  him  to  omit  this  first  irrigation  at  planting. 

TEAINING  YOUNG  TEEES 

If  walnut  trees  are  allowed  to  grow  without  any  pruning,  they  will 
usually  take  their  natural  form  of  a  medium  tall,  upright,  pyramidal 
tree,  with  a  pronounced  central  leader.  At  the  time  of  planting, 
walnut  trees  are  usually  headed  back  5  or  6  feet  from  the  ground,  and 
the  upper  lateral  bud  will  frequently  make  an  upright  limb  which 
will  take  the  place  of  the  removed  top  and  continue  the  leader  form 
of  tree.  If  this  central  branch  is  pruned  off,  however,  the  lower 
lateral  branches  will  form  a  more  open  vase-shaped  tree.  These  lower 
lateral  branches  thus  once  started,  take  all  the  strength  of  the  tree 
and  another  leader  seldom  becomes  established.  Figure  8  shows  an 
eight-year-old  tree  growing  as  a  leader  type. 

Both  the  central  leader  and  the  open  vase  types  of  trees  have  their 
k advocates.  The  advantage  of  the  leader  type  of  tree  is  primarily  in 
the  greater  strength  of  the  framework,  there  being  many  more  lateral 
limbs  distributed  along  a  greater  space  on  the  main  trunk  than  with 
the  vase-shaped  type,  and  thus  less  likelihood  of  breaking,  compared 
with  the  latter  type.  In  the  case  of  the  vase-shaped  type  the  few 
branches  originate  at  nearly  the  same  point  on  the  main  trunk  and 


Bulletin  332] 


WALNUT    CULTURE   IN    CALIFORNIA 


171 


this  centralizes  the  strain  of  supporting  the  crop.  In  the  case  of 
the  vase-shaped  trees,  it  is  not  unusual  for  one-quarter  or  one-half 
of  the  tree  to  break  off  and,  in  extreme  cases,  there  may  be  a  split  down 
the  middle  and  the  tree  be  entirely  destroyed. 


Fig.  8. — Central  leader  type  of  walnut  tree, 
of   branches,    and   plenty   of    fruiting   wood. 


This  type  has  a  strong  framework 


Figures  9  and  10  show  walnut  trees  eleven  years  old,  which  were 
trained  to  the  open  vase  shape.  It  was  necessary  to  have  from  15  to 
20  per  cent  of  the  trees  in  this  grove  braced  by  the  time  they  reached 
an  age  of  profitable  bearing.  Eventually  the  owner  will  have  to  go 
to  the  expense  of  bracing  nearly  every  tree  in  the  grove.  The  examples 
here  illustrated  may  be  considered  extreme  because  the  lateral  limbs 
originate  at  so  nearly  the  same  point. 


172  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

The  advocates  of  the  open  vase-shape  tree  claim  that  a  greater  area 
of  fruiting'  wood  is  exposed  to  the  sun  in  this  form  of  tree  than  in 
the  leader  type.  This  point  is  hard  to  verify,  and  the  greater  hazards 
of  breaking  may  offset  any  such  supposed  advantage. 

After  one  has  decided  upon  the  type  or  ideal  toward  which  the 
young  trees  are  to  be  trained,  the  pruning  operations  should  be  con- 
sistent in  following  this  initial  decision.  In  training  the  young  walnut 
tree,  very  little  pruning  is  necessary.  .Walnut  trees  as  they  are 
received  from  the  nursery  are  usually  one-year-old  whips.  If  vigorous 
8  to  12-foot  trees  are  planted  they  are  usually  cut  back  to  within  5 
or  6  feet  of  the  ground. 

Trees  thus  planted  will  frequently  start  to  grow  first  from  the 
lower  buds  within  12  to  24  inches  of  the  ground,  or  will  send  out  shoots 
along  their  entire  length.  This  growth  should  be  rubbed  off  by  going- 
over  the  trees  frequently  during  the  early  summer,  keeping  all  buds 
off  which  occur  within  4  or  5  feet  from  the  ground,  according  to  the 
form  of  tree  to  be  grown,  and  their  height  at  planting.  If  the  lower 
lateral  buds  have  made  a  growth  of  8  to  10  inches  it  is  advisable  to 
"pinch  off"  the  growing  tips  which  will  have  the  effect  of  forcing  the 
growth  of  the  upper  shoots,  while  the  growth  of  the  lower  shoots  is 
restricted.  The  smaller  lower  shoots  thus  produce  shade  and  nourish- 
ment for  the  trunk.  If  the  lower  dormant  buds  are  rubbed  off  at 
planting,  all  the  growth  will  take  place  in  the  upper  buds  without 
further  attention.  If,  on  the  contrary,  these  lower  buds  are  allowed 
to  grow,  the  buds  on  the  upper  24  inches  of  the  tree  may  remain  dor- 
mant; in  fact,  in  many  instances  the  upper  portion  of  the  tree  dies 
back  12  to  24  inches.  With  the  lower  buds  suppressed,  however,  the 
upper  buds  are  forced  into  growth  and  the  framework  of  the  tree  is 
started  in  the  upper  24  inches  of  the  trunk.15  This  is  illustrated  by 
Fig.  11. 


is  A  few  planters  advocate  cutting  the  young  tree  back  to  within  12  to  16  inches 
of  the  ground.  This  method  forces  into  growth  the  latent  buds  near  the  ground; 
one  of  these  is  selected  to  make  the  tree,  and  all  others  are  rubbed  off.  The 
selected  shoot  must  be  staked  and  tied  to  a  2"  X  2"  stake,  5  to  6  feet  tall,  other- 
wise the  supple  growth  will  be  whipped  around  and  bent  over  by  the  winds.  It 
is  claimed  that  this  method  gives  a  better  stand  of  trees,  and  less  liability  of  sun- 
burn injury  on  the  trunk.  The  tops  thus  cut  back  are  relatively  surer  to  balance 
up  with  the  root  systems  injured  by  digging.  It  may  well  be  doubted  if  the  extra 
expense  in  staking  and  in  the  care  necessary  to  grow  a  tree  in  this  manner  will 
repay  for  the  assurance  of  the  growth  of  a  larger  percentage  of  the  transplanted 
trees.  Good,  plump  nursery  trees,  if  handled  properly,  should  transplant  very 
readily  and  make  nearly  a  100  per  cent  stand  of  trees  without  resorting  to  this 
severe  cutting.  If  they  are  thoroughly  whitewashed,  or  use  is  made  of  tree  pro- 
tectors, sunburning  need  not  be  feared.  It  may  be  advisable,  however,  if  one 
is  using  inferior  trees  or  such  as  have  dried  out  in  transit,  to  cut  them  back  within 
16  inches  of  the  ground. 


Bulletin  332J 


WALNUT    CULTURE   IN    CALIFORNIA 


173 


Whether  the  ideal  selected  is  the  leader  type  or  the  open  vase 
shape,  it  is  desirable  to  select  the  lateral  branches  for  the  framework 
of  the  tree,  spaced  as  far  apart  as  practical  when  the  young  trees  are 
pruned  at  the  end  of  the  first  year's  growth. 


3ai££>;  '*"'"4V«  •  w:*:'*k 


F5#i-.d®" 


Fig.  9.— Result  of  heading  low,  starting  all  the  branches  from  one  point.  This 
could  have  been  prevented  when  the  tree  was  young  by  spacing  the  branches 
through  a  greater  distance  on  the  trunk  of  the  tree. 

With  the  vase  type  this  perpendicular  spacing  throughout  18  to  24 
inches  will  give  greater  strength  to  the  framework  than  grouping  the 


174 


UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 


branches  at  more  nearly  one  point  of  origin.  The  lateral  branches 
to  be  left  for  the  framework  of  the  tree  may  be  3  to  5  in  number,  as 
evenly  spaced  as  possible  as  they  radiate  outward  from  the  trunk. 


Fig.  10. — Method  of  bracing  open  vase  type  of  tree  to  prevent  splitting. 

The  upper  or  second  bud  from  the  top  of  the  original  tree  will  usually 
produce  a  branch  which  grows  in  a  nearly  upright  position ;  this,  of 
course,  should  be  "pinched  back"  during  the  early  summer  or  cut 


BULLETIN   332]  WALNUT   CULTURE  IN    CALIFORNIA  175 

out  during  the  dormant  season,  in  forming  the  framework  of  an  open 
vase-shaped  tree.  Figure  12  illustrates  an  example  of  this  type  of 
tree. 

In  the  case  of  the  leader  type  it  is  essential  to  space  the  laterals 
far  apart  and  not  leave  too  many;  otherwise  the  leader,  or  central 
shaft,  will  be  "choked"  out,  as  illustrated  by  Fig.  13.  Only  two  or 
three  of  the  laterals  should  be  left  in  addition  to  the  central  leader 
at  the  end  of  the  first  year,  as  other  lateral  branches  will  grow  out 
during  the  following  years  from  the  central  leader;  thus  the  shape 
of  the  trees  is  formed  during  the  first  two  or  three  years'  growth. 
The  branches  which  are  left  on  the  tree  are  not  cut  back  in  common 
practice  and  by  all  means  the  leader  should  not  be  cut  back.  Figure  8 
shows  a  desirable  shape  for  a  tree  of  this  type. 

Some  of  the  most  desirable  types  of  trees  judging  from  the  view- 
point of  strength  of  framework  and  area  of  bearing  surface,  are  a  sort 
of  modification  of  both  the  leader  and  the  vase-shaped  type.  Figure 
14  illustrates  such  a  tree;  there  is  a  pronounced  leader  through  4  or 
5  feet  of  the  main  frame  of  the  tree  which  finally  divides  up  into  a 
strong,  compact,  spreading  type  of  tree.  This  type  can  be  trained 
by  maintaining  the  leader  type  through  the  first  four  years  and  then 
during  the  dormant  season,  pruning  back  the  leader  to  a  strong  lateral 
branch  9  or  10  feet  from  the  ground.  The  tree  shown  in  the  illus- 
tration might  well  serve  as  a  model,  or  ideal,  in  training  young  walnut 
trees. 

VALUE  OF  ORCHARDS  AND  LANDS 

The  valuation  of  walnut  orchards  and  lands  suitable  for  walnut 
production  varies  considerably  in  the  different  localities  and  even  in 
the  same  districts.  Such  factors  as  suitability  of  the  soil,  water  rights, 
frost  conditions,  and  proximity  to  transportation  and  civic  centers 
are  reflected  in  the  appraised  valuation  of  walnut  properties.  The 
value  of  established  groves  may  be  greatly  influenced  by  the  variety 
of  nut,  rootstock,  planting  arrangement,  etc.  The  average  or  the 
most  general  valuations  are  all  that  can  be  presented  here. 

Replies  to  questionnaires  sent  out  by  the  California  Walnut 
Growers'  Association  in  1919  and  again  in  1921,  indicated  that  mature 
bearing  orchards  are  valued  at  about  $1400  per  acre,  while  the  young 
non-bearing  properties  are  valued  at  approximately  $900  per  acre. 
The  following  table  shows  the  exact  average  valuation  and  the  number 
of  properties  considered. 


176  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 


TABLE  IV 

Average  Valuation  of  Walnut  Groves 

(Including  water  rights) 

Year 

observations 

were  made 

Valuation 

A 

Number  of 
properties 
represented 

Bearing                               Non-bearing 

1919* 

$1299.02                          $995.00 

922 

1921f 

$1477.05                          $828.65 

231 

Land  suitable  to  walnut  culture,  plus  adequate  water  rights,  may 
be  purchased  at  from  $600  to  $1000  per  acre  in  the  proved  walnut- 
growing  sections  of  southern  California.  Such  land  may  have  a  rather 
fixed  value  for  the  production  of  beans,  beets,  or  even  citrus  fruits; 
thus  its  potential  value  for  walnut  production  is  only  one  factor  which 
determines  its  appraisement. 

In  the  central  or  northern  portion  of  the  State,  where  the  walnut 
industry  is  not  so  extensively  developed  as  in  the  south,  land  which  is 
likely  to  prove  suitable  for  walnut  culture  may  be  purchased  for  $200 
to  $300  per  acre.  This  may  not  include  a  water  right  in  any  estab- 
lished irrigation  system,  but  such  lands  occur  in  areas  where  good 
wells  are  known  to  exist. 


CULTURE 

SOIL  MANAGEMENT 
There  is  no  one  best  way  to  cultivate  all  walnut  groves.  The 
methods  of  performing  the  various  operations  of  stirring  the  soil, 
such  as  plowing,  discing,  and  harrowing,  vary  widely  in  different 
walnut  groves.  These  different  methods  may  give  equally  good  results 
in  their  respective  groves,  when  measured  by  profitable  crop  produc- 
tion. A  practice  which  leaves  one  soil  in  first-class  condition  may  be 
very  poor  practice  on  another  soil  of  a  different  type.  Most  walnut 
trees  are  deep-rooted  and  it  seems  quite  apparent  in  many  cases  that 
the  character  of  surface  soil  cultivation  is  of  only  minor  importance 
to  the  welfare  of  the  tree,  except  as  this  cultivation  affects  the  con- 
servation and  use  of  soil  moisture  by  the  tree.  There  are  many  door- 
yard  and  roadside  trees  which  produce  heavy  crops  and  yet  the  soil 
around  them  may  not  be  stirred  from  one  year's  end  to  another.  It 
does  not  follow  from  this,  however,  that  a  walnut  grove  should  not 


*  ' '  The  California  Walnut, ' »  p.  15,  1919.  Published  by  The  California  Walnut 
Growers'  Association. 

f  Brief  of  Facts  Eclating  to  the  American  Walnut  Industry,  p.  23,  1921.  Pub- 
lished by  the  Walnut  Protective  League. 


Bulletin  332 


WALNUT    CULTURE   IN    CALIFORNIA 


177 


be  plowed  or  cultivated.  There  are  frequently  many  conditions  sur- 
rounding roadside  and  dooryard  trees  which  are  favorable  to  the  pres- 
ence of  an  abundance  of  soil  moisture,  which  cannot  be  duplicated 
under  orchard  conditions.  Isolated  trees  have  the  advantages  of 
better  exposure  to  sunlight  and  a  far  greater  range  for  root  develop- 
ment and  available  soil  moisture  than  trees  planted  in  orchard  form. 


Fig.  11. — Walnut  trees  when  planted  are  usually  cut  back  to  within  5  or  6  feet 
of  the  ground. 

(a)   If  they  are  not  trained  they  will  frequently  send  out  shoots  along  their 
entire  length  during  the  early  summer. 

(&)   Frequently  if  the  lower  buds  within  two  feet  of  the  ground  are  allowed  to 
grow,  the  upper  24  inches  of  the  trees  may  remain  dormant. 

(c)   If  the  lower  buds  are  suppressed  the  upper  ones  are  forced  into  growth 
and  the  framework  of  the  tree  is  started  in  the  upper  24  inches  of  the  trunk. 


178  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

The  majority  of  walnut  groves  are  plowed  once  a  year,  usually  in 
the  very  early  spring.  A  disc,  springtooth  or  spiketooth  harrow  is 
then  used  to  put  the  land  into  good  physical  condition.  The  clean 
cultivation  during  the  summer  months  is  done  in  relation  to  the  irri- 
gation practice  and  the  conservation  of  soil  moisture. 

The  practice  of  plowing  the  groves  once  a  year  seems  essential 
to  most  of  the  soil  types  found  in  the  walnut  districts  of  California. 
The  relation  of  such  plowing  to  the  penetration  of  the  rains  into  the 
subsoils  and  the  application  of  irrigation  water  alone  justifies  the  oper- 
ation. Putting  6  to  10  inches  of  the  surface  soil  into  a  friable,  mellow 
condition  every  year  makes  it  practical  to  use  deep  irrigation  furrows 
and  thus  to  insure  deep  penetration  and  a  more  complete  use  of  the 
irrigation  water  applied  the  following  season.  An  annual  plowing 
prevents  the  tree  roots  from  becoming  permanently  established  in 
the  surface  six  inches  of  soil,  and  thus  becoming  subject  to  the  great 
variations  of  soil  moisture  content  which  occur  there  during  the 
summer  months. 

If  the  land  is  plowed  in  the  fall  and  left  rough,  the  rains  may  be 
expected  to  penetrate  deeper  into  the  subsoil  because  of  the  lessened 
run-off.  This  object  is  accomplished  best,  in  some  instances,  if  the 
plowing  is  done  at  right  angles  to  the  grade  of  the  land.  Where  a 
winter  cover  crop  is  grown,  fall  plowing  may  be  impractical  because 
of  the  necessity  of  planting  the  cover  crop  early,  either  just  before 
the  harvest,  or  immediately  after.  When  winter  cover  crops  are  not 
grown,  the  practice  of  fall  plowing  has  much  to  commend  it.  A 
medium  to  light  irrigation  may  well  precede  the  fall  plowing,  and 
the  land  left  rough  without  further  cultivation  until  the  early  spring. 
Late  spring  plowing  is  objected  to  by  some  walnut  growers  as  they 
believe  the  cutting  of  the  feeder  roots  by  such  plowing  to  be  especially 
harmful  when  the  tops  are  just  starting  to  grow,  during  late  March 
and  the  month  of  April.  The  instances  which  are  frequently  cited 
to  illustrate  the  poor  crop  conditions  following  late  spring  plowing, 
are  not  readily  diagnosed.  It  usually  seems  that  there  may  be  other 
more  important  factors  conducive  to  poor  crops,  accompanying  the 
late  plowing.  The  dry  soil  condition  which  is  so  frequently  correlated 
with  late  plowing,  may  in  itself  be  more  truly  responsible  for  the  poor 
crops  than  the  mere  season  of  plowing.  Many  orchards  which  are 
plowed  late  are  cover-cropped  or  heavily  covered  with  volunteer 
growth  of  barley,  oats,  native  annual  plants,  etc.  In  most  seasons, 
and  in  most  walnut  districts,  the  presence  in  late  March  or  early 
April  of  a  sown  cover  crop  or  a  volunteer  crop,  may  be  taken  as  a 
safe  indication  that  the  soil  moisture  is  largely  depleted,  unless  the 


Bulletin  332] 


WALNUT    CULTURE   IN    CALIFORNIA 


179 


land  has  been  thoroughly  irrigated  once  or  twice  during  the  growth 
of  the  crop.  The  orchard  in  which  a  volunteer  crop  is  allowed  to 
grow  is  most  likely  to  be  neglected  in  regard  to  winter  irrigation. 
Many  such  orchards  are  plowed  in  the  spring  when  the  annual  plants 
have  actually  wilted  for  lack  of  moisture,  or  after  recent  rains  have 
revived  them  by  moistening  the  soil  only  to  the  depth  of  the  plow 


Fig.   12. — A  typical   open-vase   type   of  tree. 


furrow.  The  subsoil  from  the  second  to  the  eighth  foot,  in  groves  thus 
managed,  may  be  nearly  dust  dry  at  this  season  when  the  trees  are 
commencing  to  leaf  out. 

The  amount  of  water  required  to  produce  a  cover  crop  is  often 
underestimated  by  the  walnut  grower.  This  is  especially  true  when 
the  crop  is  produced  during  the  rainy  season  and  makes  a  satisfactory 
growth  without  irrigation.  This  matter  is  more  fully  treated  under 
the  heading  "Cover  Crops/'    Many  such  instances  of  the  above  have 


180  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

been  noted,  when  an  examination  of  the  subsoils  has  shown  a  dryness16 
hardly  credible.  The  fact  that  the  orchard  was  being  plowed  at  the 
time  the  observation  was  made,  and  a  few  feeder  roots  were  being  cut 
in  the  late  spring,  has  seemed  to  some  observers  as  unimportant,  com- 
pared with  the  extremely  unfavorable  soil  moisture  conditions  existing 
at  the  commencement  of  the  growing  season  for  the  trees.  It  seems 
reasonably  safe  to  assert  that  the  chief  factor  contributing  to  poor 
crops  in  many  late  plowed  orchards  is  the  dry  soil  condition  during 
the  winter  and  early  spring,  and  the  late  plowing  which  may  be  asso- 
ciated with  this  dryness  is  in  no  great  degree  responsible.  Late  plow- 
ing may  be  accompanied  by  ideal  soil  moisture  conditions  as  noted 
later,  in  which  case  the  crops  may  not  suffer. 

Judging  the  question  of  late  spring  plowing  from  another  angle, 
there  are  many  first-class,  heavy  producing  orchards  which  are 
annually  spring-plowed  in  April,  when  the  leaves  are  coming  out  on 
the  trees.  Some  of  these  orchards  are  located  on  the  lowlands  subject 
to  winter  floods,  where  it  is  necessary  to  cover  crop  the  land  to  prevent 
erosion  and  where  this  threat  of  flood  damage  is  not  past  until  April. 
Orchards  in  these  districts  are  seldom  or  never  plowed  until  late,  yet 
many  of  them  produce  approximately  2000  pounds  per  acre.  Such 
plantings  are  usually  well  irrigated  during  the  spring  or  late  winter 
months,  however,  either  by  regular  irrigation  methods  or  by  the 
passage  of  flood  waters  over  the  lands. 

This  discussion  should  not  be  taken  as  an  encouragement  to  late 
spring  plowing,  as  there  are  points  in  favor  of  plowing  early  and  it 
seems  to  be  the  practice  most  usually  recommended  by  successful 
walnut  growers.  This  discussion  is  offered  mainly  to  encourage 
growers  to  feel  that  they  may  maintain  a  first-class  walnut  grove,  even 
though  for  any  reason  they  are  obliged  to  plow  late,  providing  other 
factors,  chiefly  soil  moisture  conditions,  are  favorable  to  the  early 
spring  growth  of  the  tree  and  nuts  during  March  and  April  when 
such  plowing  is  frequently  done. 

•      COVER  CROPPING 
The  practice  of  cover  cropping  walnut  groves  prevails  in  probably 
somewhat  less  than  50  per  cent  of  the  total  area  of  bearing  groves. 

is  Walnut  groves  have  been  examined  by  the  writer  in  which  the  soil  below 
the  penetration  of  winter  rains  contained  only  hygroscopic  moisture.  Walnut 
trees  commonly  reduce  the  soil  moisture  to  this  point  during  the  fall  months. 
A  winter  rainfall  of  8  to  12  inches  may  not  percolate  below  the  second  or  third 
foot  of  soil  if  there  is  a  cover  crop  or  weed  crop  growing  on  the  land.  If  the 
rains  come  in  small  amounts  distributed  over  5  or  6  months,  as  frequently  occurs, 
their  shallow  percolation  is  more  pronounced  than  when  large  volumes  fall  at  a 
time. 


BULLETIN   332]  WALNUT   CULTURE   IN    CALIFORNIA  181 

Many  groves  which  are  not  sown  to  cover  crops  produce  a  volunteer 
growth  of  various  annual  plants,  such  as  bur  clover,  lupins,  wild  oats, 
and  alfilaria,  during  the  rainy  season. 

There  is  very  little  or  no  definite  information  concerning  the 
subject  of  the  effect  of  cover  crops  on  walnut  trees.17 

Though  we  have  no  definite  field  trials  to  inform  us  concerning 
the  effects  of  cover  crops  in  walnut  groves,  a  discussion  of  some  of 
the  factors  to  be  considered  may  be  useful.  Possibly  the  primary 
factor  is  the  problem  of  supplying  sufficient  irrigation  water  for  the 
walnut  trees  and  the  cover  crop  combined.  While  it  is  true  that 
walnut  trees  do  not  draw  very  heavily  upon  the  soil  moisture  during 
the  winter  months,  yet  it  is  highly  important  to  have  the  water  content 
of  the  soil  somewhere  near  optimum  during  the  entire  winter.  It  is 
a  mistake  to  allow  a  winter  cover  crop  to  deplete  the  soil  of  its  neces- 
sary moisture.  There  should  be  no  misconception  about  the  saving  of 
soil  moisture  because  of  the  shade  of  the  cover  crop.  This  shade  and 
thus  possible  conservation  of  moisture  in  the  few  surface  inches  of 
soil,  is  hardly  worthy  of  mention,  compared  with  the  soil  moisture 
necessary  to  produce  the  cover  crop  itself. 

The  actual  water  required  to  produce  a  fair  crop  of  the  several 
plants  commonly  used  as  cover  crops  in  California,  has  never  been 
carefully  studied,  so  far  as  known  to  the  writer.  A  good  general  idea, 
however,  of  the  water  requirements  of  such  crops  can  be  obtained  from 
both  the  controlled  experiments  with  related  plants,  and  the  experi- 
ences of  farmers. 

In  studying  the  water  requirements  of  certain  plants,  Briggs  and 
Shantz18  found  that  different  plants  require  different  amounts  of 
water  to  produce  equal  quantities  of  dry  matter.19 

The  average  requirement  of  alfalfa,  field  peas,  and  melilotus  alba 
was  803  pounds,  which  is  more  than  is  needed  by  common  graius. 


i?  The  beneficial  effects  of  plowing  under  green  manure  crops,  as  measured  by 
the  crops  of  annual  plants  which  are  later  grown  on  the  same  ground,  cannot  be 
used  as  a  safe  guide  in  theorizing  on  the  effects  of  this  practice  on  walnut  trees. 
In  the  case  of  annuals,  there  is  no  competition  between  the  cover  crop  and  the 
primary  crop.  Even  with  annuals,  much  of  the  benefits  ascribed  to  the  cover  crop 
may  have  been  due  to  crop  rotation,  as  has  been  shown  by  various  experiments 
which  were  so  laid  out  as  to  measure  this  latter  factor.  Again,  the  root  zone  of 
many  of  the  annual  crops  which  have  been  used  to  indicate  the  effect  of  cover 
crops  upon  the  productivity  of  the  soil,  corresponds  more  closely  to  the  root  zone 
of  the  cover  crops  than  to  that  of  the  very  deep-rooted  walnut  trees. 

is  The  Water  Eequirement  of  Plants,  U.  S.  Dept.  of  Agric,  Bur.  of  Plant 
Indus.,  Bull.  No.  284,  1913,  p.  47. 

19  Dry  matter  is  that  portion  of  the  plant  which  remains  after  all  the  water 
has  been  evaporated  off  by  a  temperature  slightly  above  the  boiling  point  of  water. 


182  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

TABLE  V 

Water  Requirements,  Measurements  of  Crops  at  Akron,  Colorado 


Crop 

Variety- 

Water 

requirement 

(lbs.) 

Alfalfa 

Grimm 

1068 

Field  peas 

Canada 

800 

Sweet  clover 

(M.  alba) 

709 

Average  of  three  legumes  803 

In  the  absence  of  any  more  information,  we  can  use  these  results 
to  estimate  the  approximate  water  requirements  of  the  cover  crops 
used  in  walnut  groves. 

Using  this  water  requirement  of  803  pounds  of  water  as  a  guide, 
and  a  yield  of  13  tons  per  acre  as  an  average  yield,20  with  20  per  cent 
dry  matter,  the  example  works  out  as  follows : 

5200  (lbs.  of  dry  matter)  X  803  (lbs.  of  water  required  per 
lb.  of  dry  matter)  =4,175,600  (lbs.  of  water  per  acre)  = 
18.47  acre  inches  per  acre  of  cover  crop. 

This  figure  of  18.47  inches  is  based  upon  control  experiments  where 
the  crops  were  grown  in  cans  and  the  surface  evaporation  from  the 
soil  was  practically  eliminated,  whereas  under  orchard  conditions 
there  is  considerable  loss  from  surface  evaporation  while  the  young 
cover  crop  is  becoming  established  during  the  early  fall  months.  This 
figure  is,  therefore,  probably  too  low  for  orchard  conditions. 

Turning  now  to  a  practical  example :  A  cover  crop  of  yellow  sweet 
clover  (M.  indim)  was  grown  in  a  one-year-old  walnut  grove  on  the 
Citrus  Experiment  Station  grounds  during  the  winter  of  1920-21. 
The  land  was  cropped  to  barley  during  the  spring  of  1920,  and  there- 
fore contained  practically  no  soil  moisture  available  for  the  growth 
of  a  cover  crop  in  the  fall  of  the  same  year.  The  cover  crop  was 
planted  in  September,  1920,  and  plowed  under  in  April,  1921,  when 
it  was  just  past  full  bloom.  The  clover  growth  contained  some  volun- 
teer barley,  so  that  it  closely  corresponded  to  the  cover  crops  in  the 
flood  districts,  where  barley  and  clover  are  sown  together.  From  the 
date  of  planting  until  the  middle  of  March,  15.8  acre  inches  of  irriga- 
tion water  per  acre  had  been  applied  to  this  crop.  At  this  time  the 
clover  was  just  starting  into  bloom  and  the  land  was  so  dry  that  it 
could  not  be  plowed  with  a  mould-board  plow  until  after  it  was  irri- 
gated.   It  was  then  irrigated  with  7  acre  inches  per  acre.    By  April  11 

20  W.  M.  Mertz,  Green  Manure  Crops  in  Southern  California,  Univ.  of  Calif., 
Agric.  Exp.  Sta.,  Bull.  No.  292,  1918,  p.  10. 


Bulletin  332] 


WALNUT    CULTURE   IN    CALIFORNIA 


183 


the  land  was  again  too  dry  to  plow,  and  in  addition  to  the  irrigation 
water  previously  given,  7.2  inches  of  rain  had  fallen  during  the  growth 
of  the  cover  crop,  making  a  total  of  30.0  acre  inches  per  acre  which  had 
been  used  by  the  small  walnut  trees  and  by  this  mixed  cover  crop  of 
sweet  clover  and  barley  of  maximum  tonnage  growing  to  maturity.21 


Fig.  13. — The  central  leader  in  this  tree  has  been  starved  out  by  the  lateral 
branches,  all  starting  out  at  nearly  the  same  place  and  cutting  off  the  food  supply 
from  the  central  trunk. 


2i  At  this  time  the  land  received  an  additional  7  acre  inches  per  acre  and  was 
then  plowed  as  soon  as  practicable ;  the  last  7  inches  should  not  be  charged  to  the 
cover  crop,  as  it  was  largely  conserved  in  the  soil  by  prompt  plowing,  leaving  the 
soil  in  a  good  condition  for  the  young  trees  and  the  planting  of  a  summer  intercrop 
of  beans. 


184  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

If  the  crop  had  been  plowed  just  as  the  clover  came  into  bloom 
following  promptly  the  March  irrigation22  there  would  have  been  15.8 
acre  inches  per  acre  irrigation  water  and  7.2  inches  of  rain,  making 
23  inches  as  a  total  used  by  the  crop  up  to  this  period  in  its  growth, 
and  leaving  the  soil  quite  as  dry  as  it  was  the  fall  before.23  The 
stage  of  development  of  the  crop  at  this  time,  just  beginning  to  bloom, 
would  have  been  the  ideal  condition  for  plowing  under.  The  tonnage 
was  nearly  at  its  maximum,  and  the  green  succulent  condition  of  the 
stalks  of  both  the  clover  and  the  barley  would  have  probably  assured 
a  more  rapid  disintegration  after  plowing  than  turning  it  under  in 
April,  when  the  flowering  season  was  practically  past. 

From  the  two  examples  cited,  it  seems  probable  that  the  growth 
of  a  legume  cover  crop  in  a  walnut  grove  should  be  expected  to  require 
from  20  to  24  acre  inches  per  acre  of  irrigation  water,  aside  from  the 
needs  of  the  trees.  The  winter  rains  alone  are  frequently  no  more 
than  sufficient  to  meet  the  requirements  of  the  trees  under  clean 
cultural  conditions.  In  practice  this  will  mean  from  3  to  4  heavy 
irrigations  during  the  winter  and  early  spring,  in  localities  receiving 
only  10  to  15  inches  of  winter  rainfall.  (See  "Irrigation.")  It  is 
sufficient  here  to  remind  the  reader  again  that  there  should  be  ample 
water  available  to  grow  the  winter  cover  crop,  and  that  following  the 
plowing  in  of  the  crop,  the  walnut  trees  should  be  given  a  thorough 
irrigation  at  the  commencement  of  their  early  spring  growth.  If 
the  requirements  of  the  cover  crop  alone  are  considered,  at  the  time 
of  plowing  the  subsoil  may  be  entirely  too  dry  to  promote  an  early 
vigorous  growth  of  the  trees  and  nuts,  even  though  the  soil  is  moist 
enough  to  plow  with  a  mould-board  plow. 

One  of  the  benefits  derived  from  the  use  of  cover  crops  in  walnut 
groves  has  been  the  improvement  of  the  physical  condition  of  the 
heavy  soils  as  a  result  of  the  incorporation  of  large  amounts  of  organic 
matter.  The  experiences  of  many  observing  farmers  seem  to  agree 
that  the  clay  loams  are  made  more  friable  and  the  irrigation  water 
penetrates  them  much  more  readily  when  cover  cropping  is  practiced. 
The  latter  point,  with  reference  to  the  deeper  penetration  of  the  irri- 
gation water,  may  alone  justify  the  use  of  cover  crops  on  certain  soils. 

The  legume  cover  crops  most  commonly  grown  in  walnut  groves 
during  the  winter  months  are  bur  clover,  yellow  sweet  clover  (Melilotus 
indioa),  purple  vetch,  and  horse  beans. 


22  It  was  too  dry  to  plow  before  this  irrigation,  as  noted. 

23  The  crop  wilted  badly  before  the  March  irrigation  and  the  soil  was  judged 
to  be  between  the  wilting  point  and  the  hygroscopic  point. 


Bulletin  332] 


WALNUT    CULTURE   IN    CALIFORNIA 


185 


The  seed  may  be  drilled  in,  or  sown  broadcast,  and  then  "  brushed 
in"  or  harrowed  lightly  with  a  spike-tooth  harrow.  The  amount  of 
seed  required  per  acre  is  as  follows : 

Melilotus  indica  and  bur  clover 20-  30  lbs.  per  acre 

Vetch 70-  80  lbs.  per  acre 

Horse  beans  100-200  lbs.  per  acre 


Fig.  14. — One  of  the  most  desirable  types  of  walnut  tree,  combining  the  good 
points  of  both  the  leader  and  vase  shape  types. 

It  is  essential  to  plant  the  cover  crop  as  early  in  the  fall  as  possible 
after  harvesting  the  nuts,  in  order  that  it  may  make  a  good  start  before 
the  cold  weather  commences,  and  thus  make  sufficient  tonnage  by  the 
middle  of  March  to  be  worth  plowing  under.  The  cover  crop  grows 
successfully,  sometimes,  if  sown  at  the  time  the  land  is  leveled  off, 
preceding  harvest,  and  the  furrowing  out  and  irrigating  of  the  land 
postponed  until  the  harvesting  operations  are  over. 

Figure  15  shows  a  good  crop  of  yellow  sweet  clover  (M.  indica) 
ready  to  turn  under  in  late  March,  while  Fig.  16  shows  a  similar  cover 
crop  being  plowed  deeply  after  first  laying  the  clover  down  with  a 
drag.  Cover  crops  are  occasionally  disced  under,  as  shown  by  Fig.  17. 
This  is  not  so  desirable  as  deep  plowing,  because  it  does  not  incorpo- 
rate the  organic  matter  in  the  soil  mass  so  closely  to  the  tree  roots.  A 
cover  crop  disced  under  improves  the  physical  condition  of  the  surface 
soil,  but  it  does  not  supplement  the  nlant  food  of  the  soil  so  much  as 
though  it  were  plowed  under. 


186 


UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 


IRRIGATION 
Before  discussing  the  irrigation  of  any  crop,  it  is  well  to  understand 
the  extent  and  distribution  of  the  root  system  of  the  crop  in  question. 
It  is  a  commonly  held  opinion  that  walnut  trees  are  very  deep-rooted 
when  grown  on  the  majority  of  the  orchard  soils  of  southern  Cali- 
fornia. There  are  exceptions  to  this  general  rule,  but  in  the  deep 
friable  soils  of  the  typical  walnut  sections  the  problem  usually  is 
not  how  deep  do  the  roots  go  but  where  do  they  stop?  In  treating 
the  matter  of  irrigation,  the  following  discussion  may  be  taken  to 
apply  to  the  volume  of  soil  included  in  the  first  eight  feet  from  the 


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Fig.  15. — A  cover  crop  of  yellow  sweet  clover  (M.  indica)  ready  to  plow  under 
in  late  March. 

surface  of  the  ground.  The  discussion  is  confined  to  this  depth  of 
eight  feet,  not  because  it  is  judged  that  this  is  necessarily  the  absolute 
limitation  of  the  root  system,  but  because  observations  have  shown : 
first,  that  the  root-feeding  area  in  many  groves  extends  to  at  least  this 
depth ;  second,  that  first-class  crops  can  be  grown  when  only  the  first 
eight  feet  of  soil  are  considered;  and  third,  if  more  of  the  walnut 
growers  observed  and  frequently  examined  this  volume  of  soil,  they 
might  be  able  to  improve  their  irrigation  practice. 

From  early  spring  until  harvest,  it  is  essential  to  have  adequate 
soil  moisture  available  throughout  the  root  zone,  if  a  good  tonnage 
of  well-filled,  heavy-shelled,  well-sealed  nuts  is  to  be  produced. 

In  discussing  the  practical  use  of  irrigation  water  in  producing 
walnuts,  perhaps  the  subject  can  be  most  clearly  presented  if  the  water 


Bulletin  332 


WALNUT    CULTURE   IN    CALIFORNIA 


187 


requirements  of  the  crop  are  considered  from  the  beginning  of  the 
growing  season  in  the  spring,  through  the  summer  and  harvest  seasons, 
and  on  to  the  following  spring. 


Fig.   16. — Method  of  turning  under  cover  crop  by  means  of  a  tractor  and  a 
mould-board  plow,  after  the  cover  crop  has  been  laid  down  with  a  drag. 


Unlike  most  other  fruit  crops,  the  walnut  is  grown  for  the  seed  of 
the  plant  instead  of  for  any  edible,  fleshy  portion  which  surrounds 
the  seed.    Experience  in  growing  stone  fruits,  such  as  peaches,  shows 


188  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

that  the  size  of  the  fruit  may  be  greatly  influenced  by  irrigation 
water  applied  late  in  the  seasonal  growth  of  the  fruit,  long  after  the 
pit  has  hardened.  This  late  increase  in  the  size  of  the  fruit  is  caused 
by  the  increased  thickness  of  the  fleshy  or  edible  portion  of  the  fruit, 
without  any  increase  in  the  size  of  the  pit  itself.  The  growth  of  the 
walnut  compares  with  the  growth  of  the  pits  of  the  stone  fruits,  all 
of  which  reach  their  full  size  long  before  harvest  and  before  the  shell 
is  fully  hardened.  The  walnut  shells  of  most  of  the  varieties  begin 
to  harden  about  the  middle  or  last  of  June.  Therefore  any  cultural 
or  irrigation  practice  which  is  intended  to  affect  the  size  of  the  nut 
must  take  place  before  that  time. 


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Fig.  17. — Discing  a  cover  crop  under  improves  the  physical  condition  of  the 
surface  soil,  but  it  is  probably  not  so  advisable  as  plowing  it  under  deeply,  thus 
bringing  it  in  closer  contact  with  the  surface  roots  of  the  trees.  • 

The  effect  of  early  irrigation  of  walnut  trees  upon  the  size  of  the 
nuts  was  shown  on  the  experimental  plots  in  Hemet,  where  the  Experi- 
ment Station  and  the  California  Walnut  Growers'  Association  Field 
Department  have  been  investigating  the  causes  of  die-back  of  walnut 
trees.  In  these  trials,  Plot  A  received  5.7  acre  inches  per  acre  more 
winter  irrigation  than  Plot  D.  They  were  otherwise  handled  the  same. 
In  both  cases,  summer  irrigation  was  commenced  in  the  middle  of 
June.  The  abundance  of  early  soil  moisture  from  the  winter  irriga- 
tion in  Plot  A  produced  a  quick  growth  of  sizable  nuts,  while  the 
deficient  early  moisture  in  Plot  D  prevented  the  nuts  from  developing 
to  normal  size.  The  average  size  of  a  sample  of  the  nuts  from  Plot  A 
was  23  per  cent  larger,  11  per  cent  heavier,  and  they  contained  27 


Bulletin   332]  WALNUT   CULTURE   IN    CALIFORNIA  189 

per  cent  more  edible  kernels  by  weight,  than  the  average  of  Plot  D. 
The  curves  showing  the  volume  measurement  of  the  nuts  from  the 
respective  plots,  expressed  in  cubic  centimeters,  is  shown  in  Fig.  18. 
The  application  of  winter  irrigations  in  the  southern  counties  as 
a  preventive  of  winter  injury  to  the  trees  will  be  more  fully  developed 
later  in  this  section.  This  winter  irrigation  serves  also  to  supply 
needed  early  spring  moisture  for  the  growth  of  the  trees.  Aside  from 
the  small  amount  of  moisture  transpired  by  the  dormant  trees  and 
lost  through  the  surface  evaporation  of  the  soil,  the  big  bulk  of  the 

Cubic  centimeters. 

13  5  7  9  11  13         15         17         19         12 

i  i 

45 

t» 
J  40 

m  35 

03 

ft30 

1  25 
o  20 
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Plot  D.  Plot   A. 

Fig.  18. — Showing  the  variation  in  size  of  150  nuts  each,  from  Plot  D  and 
Plot  A  (Hemet).  The  latter  was  winter  irrigated  5.7  acre  inches  per  acre  more 
than  the  former. 

winter  irrigation  water  is  available  for  the  trees'  use  in  the  spring, 
providing  it  is  not  largely  used  by  cover  crops  or  weeds  before  the 
trees  begin  to  grow.  During  seasons  of  normal  rainfall  this  irrigation 
may  not  be  needed  in  the  coastal  districts  from  Saticoy  northward  to 
Santa  Kosa,  but  during  seasons  of  light  rainfall,  especially  in  the 
inland  districts,  it  will  increase  the  percentage  of  No.  1  nuts  in  most 
years.  A  grove  being  thoroughly  winter  irrigated  is  shown  in  Figs.  19 
and  21.  (The  illustrations  are  presented  primarily  to  show  different 
methods  of  applying  water.) 

The  relation  of  the  winter  rainfall  and  of  the  consequent  early 
spring  soil  moisture  to  the  grade  of  the  walnut  crop  the  following 
season,  is  shown  in  Fig.  20. 


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190 


UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 


The  curves  are  projected  on  a  percentage  basis.  The  base  line 
marked  100  in  the  left-hand  margin  may  be  taken  to  represent  the 
average  of  the  normal  rainfall  of  the  following  stations :  Los  Angeles, 
Santa  Barbara,  Pomona,  and  Tustin.  At  the  same  time,  it  represents 
the  percentage  of  No.  1  nuts  sold  by  the  California  "Walnut  Growers' 
Association  in  the  total  volume  handled  during  the  six  years  noted. 
The  dotted  line  shows  the  fluctuation  of  the  percentage  of  seasonal 
(July  to  June)  rainfall,  ending  June  30,  for  the  respective  years 
indicated  on  the  bottom  margin.  The  solid  line  represents  the  fluctua- 
tion of  the  percentage  of  No.  1  nuts  handled  by  the  same  Association 
during  the  respective  years,  expressed  as  a  per  cent  of  the  grade  of 
No.  1  nuts  for  the  total  six  years'  crop. 


Fig.  19. — If  the  water  will  move  into  the  soil  readily  the  use  of  deep  furrows 
and  the  wetting  of  as  little  of  the  surface  soil  as  possible  will  reduce  the  moisture 
loss  by  evaporation  to  a  minimum. 


Although  the  curves  may  not  be  considered  parallel,  they  do  tend 
to  move  in  the  same  direction,  except  during  the  year  1917-18.  There 
are  many  factors  besides  total  rainfall  which  operate  to  affect  the 
soil  moisture  available  for  tree  and  crop  growth  during  the  early 
spring.  Among  these  are  distribution,  amount  per  storm,  weather 
conditions  between  storms,  and  season  of  rainfall,  together  with  the 
demands  upon  the  moisture  by  cover  crops  in  many  cases.  These  may 
all  bear  more  or  less  upon  the  percolation  of  the  water  into  the  soil 
and  thus,  finally,  the  amount  available  to  the  walnut  tree.     Again, 


BULLETIN   332]  WALNUT   CULTURE  IN    CALIFORNIA  191 

many  walnut  growers  practice  winter  irrigation.  In  such  cases,  the 
volume  of  rain  may  not  be  a  limiting  factor.  It  is  hardly  to  be 
expected,  then,  that  the  lines  would  be  parallel  during  the  entire 
period ;  the  fact  that  they  tend  in  the  same  direction  the  majority  of 
the  seasons  represented  is  unmistakable.  The  fact  that  the  lines  cross 
between  1917  and  1918  may  be  due  to  the  especial  effectiveness  of 
the  small  volume  of  rain  in  1918  in  percolating  deeply  into  the  soil. 
During  the  1917-18  rainy  season,  the  rains  came  late  and  extended 
over  a  short  period,  percolating  to  practically  twice  the  depth  in  the 


160 
150 
140 
130 

-  \ 

-  \ 

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i       i     .1 — I — 

120 

- 

\ 

110 



.-- 

— •-.    V. 

100 
90 
80 

\  ■ 

70 

■ 

i 

.         1         1 

1914 

1915 

1916      1917       1918       19 

Rainfall  (percentage  of  normal.) 

No.  1  Nuts  (percentage  of  six-year 

average  per  cent  grade  J. 

Fig.   20. — Expressed  in  percentages. 

same  soil  as  the  1918-19  rains.  During  1918-19  the  periods  between 
rains  were  characterized  by  northerly  winds  and  low  humidity,  making 
the  rains  much  less  effective  than  in  the  previous  year.  The  trees  may 
have  made  use  of  nearly  twice  as  much  natural  moisture  during  the 
season  following  the  1918  rains  as  during  1919,  owing  to  its  distri- 
bution over  a  shorter  season  during  the  former  year. 

The  chief  thing  that  ma3r  be  learned  from  this  is  the  value  of  soil 
moisture  in  the  early  spring  in  producing  large  nuts.  If  this  is  a 
correct  analysis  of  the  facts  presented,  the  practice  of  winter  irriga- 
tion of  walnut  groves,  which  is  followed  to  a  great  degree  in  some 
districts,  should  become  a  universal  practice  during  winters  of  scanty 
rainfall.  ' 


192  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

If  the  winter  rains  plus  the  winter  irrigation  do  not  percolate  to 
a  depth  of  eight  feet  during  the  winter  and  late  spring,  it  is  advisable 
to  apply  the  first  spring  irrigation  sometime  in  April.  At  this  season 
of  the  year  the  trees  are  leafed  out  and  the  young  nuts  should  be 
rapidly  increasing  in  size.  Filling  the  subsoil  with  moisture  at  this 
season  makes  a  reservoir  for  the  tree  roots  to  draw  upon  during  the 
early  part  of  the  summer,  while  the  nuts  are  reaching  their  full  size. 

The  soil  moisture  due  to  early  spring  irrigation,  winter  rains,  and 
winter  irrigation,  should  be  supplemented  by  water  applied  in  mid- 
summer to  promote  the  growth  of  well-filled  nuts  with  plump  kernels. 
This  may  require  three  applications  of  water  in  June,  July  and 
August,  respectively;  or  possibly  only  the  first  and  last  month  men- 
tioned, according  to  the  type  of  soil  and  the  climatic  conditions.  The 
light  sandy  soils  will  require  more  frequent  irrigation  than  the  heavier 
soils,  while  the  walnut  groves  in  the  hot  inland  valleys  will  require 
more  water  than  those  on  the  coast,  which  are  frequently  bathed  in 
heavy  fogs  and  where  the  daily  temperature  is  relatively  low.  De- 
tailed directions  for  the  irrigation  of  walnut  groves  cannot  be  pre- 
sented in  a  general  treatment  of  an  industry  which  is  found  under 
such  widely  divergent  conditions.  Emphasis  should  be  laid,  however, 
upon  the  need  of  a  continuous  supply  of  soil  moisture  from  the  time 
the  tree  leafs  out  in  the  spring,  up  to  and  including  the  beginning 
of  the  harvest  period.  Just  as  the  early  spring  moisture  is  essential 
to  the  production  of  large  nuts,  and  the  midsummer  water  to  the  full 
development  of  the  kernels,  ample  soil  moisture  in  early  September 
is  necessary  to  promote  the  normal  development  of  the  nut  to  the 
point  where  the  husk  cracks  open  and  the  nut  drops  free  to  the  ground, 
leaving  the  husks  temporarily  attached  to  the  twigs.  The  absence  of 
sufficient  water  during  the  final  development  of  the  crop  is  almost 
invariably  associated  with  the  sticking  of  the  husks  to  the  nuts,  sun- 
burning,  and  a  consequent  high  percentage  of  cull  nuts.  The  shells 
of  the  nuts  which  crack  out  of  the  husks  naturally,  are  mostly  bright 
amber  color  and  free  from  stains,  while  those  to  which  the  husk 
adheres  are  often  stained,  and  may  even  be  classified  as  culls,  if  the 
stain  is  too  deep  to  be  obliterated  by  the  bleaching  at  the  packing 
house.  The  cracking  of  the  hulls  when  plenty  of  late  summer  water 
is  applied  is  of  the  same  nature  as  the  splitting  of  prunes  when  the 
trees  are  watered  as  the  crop  is  ripening. 

It  should  therefore  be  the  aim  of  the  walnut  grower  to  keep  all  the 
soil  from  the  surface  to  a  depth  of  eight  feet  well  supplied  with 
moisture  from  early  spring  until  early  fall. 


Bulletin  332] 


WALNUT   CULTURE   IN    CALIFORNIA 


193 


It  is  a  common  observation  of  practical  walnut  growers  that  it  is 
more  difficult  to  obtain  deep  percolation  of  the  irrigation  waters  in 
midsummer  than  in  spring.  This  condition  may  be  largely  due  to  the 
fact  that  subsoils  are  allowed  to  become  too  dry  before  irrigation  in 
midsummer.  This  excessively  dry  soil  resists  the  entrance  of  moisture 
to  a  much  greater  degree  than  does  a  soil  which  is  moderately  damp 
at  the  time  irrigation  water  is  applied.  This  phenomenon  of  the  move- 
ment of  soil  moisture  into  soils  of  different  moisture  content  has  been 
observed  many  times,  both  in  the  laboratory  and  the  field. 


Warn 

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8 

Fig.  21. — This  system  of  the  dike  and  check  furrow  irrigation  is  used  to  prevent 
"run  off"  and  obtain  a  deep  moisture  penetration. 

Field  observations  on  plots  D  and  E  of  the  Hemet  irrigation  experi- 
ment harmonize  with  laboratory  studies  which  have  been  made  of 
this  subject.  The  sandy  loam  subsoil  (fourth  to  seventh  foot,  inclu- 
sive) of  D  was  extremely  dry  (practically  the  hygroscopic  point)  as 
early  in  the  season  as  May  1,  while  the  same  volume  of  subsoil  of  plot 
E  reached  onl}T  a  dryness  which  might  be  considered  in  harmony  with 
good  irrigation  practice  (somewhat  above  the  point  where  most  annual 
plants  would  wilt)  by  the  first  of  June.  The  plots  are  adjacent  to  each 
other  and  were  both  irrigated  in  the  same  manner  and  given  the  same 
amount  of  water  during  June,  namely,  4.2  acre  inches  per  acre.  The 
average  penetration  of  the  same  amount  of  irrigation  water  applied 
on  these  two  plots  was  as  follows:  D,  53  inches;  E,  84  inches.  Figure 
21  illustrates  an  orchard  being  irrigated  by  the  dike  and  check-furrow 
system.  This  prevents  "run-off"  when  a  large  volume  of  water  is 
used  and  secures  a  deep  moisture  penetration. 


194  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

The  lesson  which  may  be  learned  from  the  above  or  similar  obser- 
vations points  to  the  necessity  of  applying  irrigation  water  before  the 
subsoils  become  excessively  dry,  if  a  deep  penetration  of  the  water  is 
desired.  Just  when  this  time  arrives  depends  upon  the  cultural  and 
irrigation  practice  which  has  prevailed  during  the  winter  and  early 
spring.  If  the  land  has  not  been  winter  irrigated,  or  if  a  cover  crop 
has  been  grown  without  adequate  water,  the  subsoil  may  already  be 
dried  out  below  the  wilting  point  by  April  1.  It  is  impossible  to  lay 
down  any  hard  and  fast  general  rule  without  examining  the  soil,  and 
this  brings  us  to  the  next  point  we  wish  to  make :  Every  walnut  grower 
should  study  and  frequently  observe  the  moisture  condition  of  the 
subsoil  as  well  as  the  surface  soil  in  the  walnut  grove. 

The  subsoils  can  be  examined  and  readily  sampled  by  means  of 
a  soil  auger.  Such  a  tool  can  be  made  by  any  blacksmith  and  may 
consist  of  a  two-inch  carpenters'  auger,  welded  to  a  steel  rod,  or  a 
half-inch  gas  pipe.  For  the  intelligent  irrigation  of  a  deeprooted 
crop  like  walnuts,  a  soil  auger  is  as  much  a  necessary  part  of  the 
equipment  as  an  irrigators'  shovel.24  Opposite  sides  of  a  given  ten- 
acre  tract  may  take  water  quite  differently  in  the  subsoil,  while  the 
surface  soil  seems  uniformly  irrigated.  Only  by  the  use  of  a  soil 
auger  can  the  farmer  make  any  more  than  a  poor  guess  at  the  soil 
moisture  conditions  throughout  the  deep  root  zone  of  the  walnut  tree. 
Figure  22  shows  a  farmer  examining  the  subsoil  in  his  grove  to  a 
depth  of  8  feet,  for  the  first  time.  Enough  was  learned  from  this 
one  examination  to  convert  him  to  the  use  of  the  auger  henceforth. 

The  amount  of  water  necessary  to  wet  down  through  the  root  zone 
in  a  walnut  grove  will  depend  largely  upon  the  type  of  soil  and  the 
degree  of  dryness  which  prevails  before  the  water  is  applied.  In  gen- 
eral, it  will  require  from  l1/^  to  iy2  acre  inches  per  acre  for  each  foot 
depth  of  soil,  to  bring  the  moisture  from  the  wilting  point  or  below, 
up  to  the  normal  field  moisture-holding  capacity  of  the  soil.  This 
may  be  taken  as  only  a  very  general  rule,  however,  as  a  clay  loam 
soil  which  has  been  allowed  to  become  very  dry  may  require  more 
than  the  above,  while  a  sandy  loam  only  moderately  dried  will  require 
less.  In  any  case,  the  farmer  should  study  closely  the  movement  of 
the  irrigation  waters  in  his  own  soils. 

If,  for  example,  he  wishes  to  apply  water  enough  on  a  sandy  loam 
soil  to  penetrate  downward  8  feet  from  the  surface,  it  may  require 
10  acre  inches  per  acre  (1*4  X  8  =  10)  if  the  subsoil  is  dry,  as  well 


24  A  soil  auger  has  been  designed  by  the  Orchard  Management  Division,  and 
is  sold  to  walnut  growers  at  cost  by  the  California  Walnut  Growers'  Association. 


Bulletin  332 


WALNUT    CULTURE   IN    CALIFORNIA 


195 


as  the  surface  soil,  at  the  beginning  of  the  irrigation  run.  On  the 
other  hand,  6  acre-inches  may  be  ample  if  the  subsoil  is  not  excessively 
dried.  On  land  underlaid  by  clay  or  gravel  within  8  feet  from  the 
surface,  there  is  a  likelihood  that  the  root  zone  is  correspondingly 
shallower  than  in  the  preceeding  example. 


Fig.  22. — A  farmer  examining  the  subsoil  in  his  grove  to  a  depth  of  8  feet. 

If  the  distribution  of  the  root  system  of  a  crop  justifies  the  practice 
of  applying  water  sufficient  to  wet  down  to  a  depth  of  8  feet,  such  a 
use  of  water  at  infrequent  intervals  will  be  more  economical  than 
frequent  shallow  irrigations.  In  the  former  case  a  smaller  percentage 
of  the  total  water  applied  will  be  lost  by  surface  evaporation  in  the 
first  8  or  10  inches  of  soil.  The  frequency  of  irrigations  desirable 
will  depend  upon  the  age  and  the  number  of  trees  per  acre,  the  soil 


196  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

type,  the  presence  of  intercrops,  climatic  conditions,  etc.,  all  of  which 
comes  back  again  to  the  necessity  of  each  farmer  ''going  to  the  bot- 
tom" of  his  own  soil  conditions  with  a  soil  auger. 

Measurement  of  the  water  applied  (explained  on  page  150)  is  as 
important  as  studying  its  movement  through  the  subsoils  to  the 
farmer  who  wishes  to  learn  the  most  about  the  moisture  conditions  of 
his  soil  and  to  improve  his  knowledge  of  the  use  the  walnut  trees  are 
making  of  the  water  applied.  It  may  prove  a  greater  mistake  in  the 
end  to  apply  too  much  water  than  not  to  apply  enough.  "Water 
applied  in  excess  is  lost  in  the  country  drainage  and  may  contribute 
to  the  rise  of  seepage  water,  to  the  eventual  detriment  to  lands  lower 
down,  if  not  to  the  land  in  question.  This  may  seem  a  far  cry  to 
many  people  who  own  groves  where  the  ground  water  is  now  30  to 
50  feet  from  the  surface.  Such  a  rise  of  ground  or  seepage  waters 
has  occurred,  however,  through  many  valleys  of  the  West  where  the 
ground  waters  were  formerly  30  to  40  feet  from  the  surface.  Much, 
of  course,  depends  upon  the  nature  of  the  deep  soil  formations.  It 
is  sufficient  now  to  say  that  the  farsighted  intelligent  irrigator  who 
looks  upon  his  walnut  grove  as  a  permanent  investment,  will  study 
the  movement  of  soil  moisture,  not  only  throughout  the  root  zone, 
but  farther,  and  inquire  into  the  possible  loss  of  water  by  deep  perco- 
lation beyond  the  reach  of  the  walnut  trees. 

The  actual  method  of  applying  the  water  to  the  soil  must  be 
adapted  to  the  conditions  of  each  case.  Three  different  systems  of 
applying  water  are  illustrated  by  Figs.  19,  21,  23,  24,  and  25.  The 
advantages  of  the  respective  methods  are  briefly  stated  under  each 
illustration. 

INTERCROPPING 

Intercropping  the  young  walnut  groves  is  a  practice  which  pre- 
vails in  a  large  majority  of  cases.  This  practice  has  usually  proved 
very  successful  in  making  the  land  support  the  orchard  before  the 
walnuts  come  into  profitable  bearing. 

.  The  best  intercrop  to  use  will  depend  somewhat  upon  local  circum- 
stances. In  the  bean  districts  and  in  cases  where  the  walnut  groves 
are  large  enough  to  justify  the  maintenance  of  bean  farm  machinery, 
beans  are  an  ideal  intercrop  for  the  young  walnut  grove.  The  bean 
is  a  legume  and  requires  thorough  cultivation.  These  facts  contribute 
to  the  best  conditions  for  the  development  of  the  young  trees,  if  suf- 
ficient irrigation  water  is  avaialble  for  both  crops.  The  bean  straw 
can  be  plowed  under,  which  is  an  advantage  of  the  bean  intercrop 
over  all  others. 


Bulletin  332] 


WALNUT    CULTURE   IN    CALIFORNIA 


197 


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Fig.  23. — The  furrow  system  is  probably  the  most  advisable  way  of  applying 
water,  if  the  water  will  move  into  the  subsoil  readily  and  the  surface  slope  is  such 
that  it  flows  across  the  orchard  satisfactorily,  and  without  much  ' '  run-off. ' ' 


Fig.  24. — If  the  land  is  check-furrowed  at  the  lower  half  of  the  orchard,  the 
run-off  will  be  reduced  and  added  pressure  of  deeper  water  in  the  furrow  forces 
the  moisture  into  the  dry  subsoil. 


198  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

Outside  of  the  bean  districts,  the  choice  of  intercrops  largely 
depends  upon  the  market  conditions  for  the  proposed  crops.  Several 
of  the  vegetables  commonly  grown  for  canning  factories  may  be  profit- 
ably grown  in  the  young  orchard  without  harm  to  the  trees.  Such 
crops  as  tomatoes,  peppers,  and  string  beans  are  often  used.  Figure  26 
illustrates  a  bean  intercrop  in  a  bearing  walnut  grove. 

Intercrops  of  corn,  milo,  squashes,  and  pumpkins  are  not  so  well 
thought  of  among  many  experienced  walnut  growers,  because  of  their 
apparently  harmful  effect  upon  the  trees.  Milo  especially  is  generally 
considered  a  bad  intercrop  for  walnuts.  Milo  is  a  very  drought- 
resistant  crop  with  a  relatively  low  water  requirement.  It  will  there- 
fore develop  a  first-class  crop  under  soil  conditions  unfavorable  to 
the  best  growth  of  the  walnut  trees.  The  thrifty  appearance  of  the 
milo  may  deceive  the  farmer  concerning  the  condition  of  the  walnut 
roots.  Only  the  closest  students  of  irrigation  should  attempt  to  grow 
this  crop  in  a  walnut  grove,  and  it  may  be  of  doubtful  value  even 
then. 

If  the  grower  has  only  a  small  plantation,  not  justifying  the 
maintenance  of  bean  farm  machinery,  and  has  not  a  ready  market  for 
a  vegetable  crop,  the  intercropping  with  a  precocious  bearing  fruit  tree 
may  be  desirable.  Peaches,  prunes,  and  apricots  are  among  the  fruit 
trees  most  commonly  used  in  such  schemes  of  intercropping.  In  many 
instances  observed,  the  growing  of  these  stone  fruits  in  a  young  walnut 
grove  has  retarded  the  growth  of  the  nut  trees,  when  compared  with 
groves  interplanted  with  beans.  Nevertheless,  in  many  cases,  the 
fruit  trees  have  been  profitable  and  have  made  the  orchard  self- 
supporting  while  the  nut  trees  were  growing,  thus  out-weighing,  from 
a  business  point  of  view,  the  temporary  check  of  the  walnut  trees. 
If  fruit  trees  are  used  as  an  intercrop,  the  farmer  must  study  his 
irrigation  problem  carefully  to  be  sure  the  walnut  trees  are  not  robbed 
of  their  needed  water  by  the  companion  crop.  General  observations 
indicate  that  the  apricot  tree  is  a  keener  competitor  of  the  walnut 
than  the  peach,  and  thus  less  suitable  to  plant. 

The  disadvantages  of  intercropping  the  young  walnut  grove  with 
fruit  trees  should  be  considered  before  laying  out  a  plantation  in 
this  manner.  The  following  are  among  the  most  serious  objections. 
If  the  cling-stone  canning  peaches  are  used  as  an  intercrop,  they  will 
require  late  summer  irrigation  to  fully  develop  their  crop,  after  water- 
ing should  be  discontinued  upon  the  walnuts.  This  discordant  water 
requirement  will  be  most  troublesome  when  the  plantation  is  from 
three  to  six  years  old.    At  this  period  the  owner  is  naturally  anxious 


BULLETIN   332]  WALNUT   CULTURE   IN   CALIFORNIA 


199 


Fig.  25. — Basin  irrigation,  with  water  standing  six  or  seven  inches  deep  over 
the  land,  insures  its  movement  into  the  subsoil.  This  is  especially  practical  on 
the  level  sandy  soils.  This  system  may  also  be  used  to  advantage  where  the  surface 
soils  are  very  sandy  and  thus  take  water  so  rapidly  that  it  is  impractical  to  force 
a  small  stream  in  a  furrow  across  the  orchard. 


Fig.  26. — Showing  beans  used  as  a  profitable  intercrop  in  a  ten-year-old  grove, 
in  a  bean-growing  district. 


200  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

to  make  the  most  of  the  first  commercial  peach  crops,  and  some  of 
the  latest  and  most  profitable  varieties  should  be  watered  in  late 
August  and  September  in  order  to  bring  them  to  a  good  size  for  the 
canning  factory.  This  late  water  on  young  walnut  trees  is  very  liable 
to  keep  the  twigs  growing  so  late  that  they  will  be  severely  injured 
by  the  early  fall  frost.  Such  a  hazard  will  diminish  as  the  walnut 
trees  come  into  bearing  and  the  drain  of  crop  production  acts  as  a 
regulator  in  slowing  up  the  late  summer  growth  and  in  thus  aiding 
the  walnut  trees  to  become  nearly  dormant  at  the  season  of  early  fall 
frosts.  The  prune  has  been  a  profitable  intercrop  with  walnuts,  but 
here  the  discordant  water  requirements  are  somewhat  reversed,  and 
do  not  become  pronounced  until  both  the  prunes  and  the  walnuts  are 
in  bearing.  At  this  stage  in  the  development  of  the  plantation,  when 
it  is  possibly  from  six  to  twelve  years  old,  and  before  it  seems  econom- 
ically sound  to  pull  out  the  prunes,  the  late  summer  water  require- 
ments are  quite  opposite  with  these  two  crops.  As  the  walnuts  mature 
they  should  be  irrigated  so  that  they  will  be  fully  developed  and  so 
that  the  husks  will  crack  open,  producing  a  clean-shelled,  plump- 
kerneled  nut ;  irrigation  of  the  prunes  at  this  time  will  also  cause  them 
to  crack  open  and  in  so  doing  make  an  inferior  product,  liable  to 
rejection  by  the  prune-selling  agencies. 

If  either  prunes  or  peaches  are  used  as  interplants  it  would  seem 
advisable  to  plant  the  walnuts  30'  X  60'  and  the  fruit  trees  in  solid 
rows  in  the  middle  of  the  interspaces.  These  temporary  trees  might 
be  planted  only  15  feet  apart  in  the  row,  which  would  give  enough 
fruit  trees  per  acre  to  make  it  worth  while,  and  yet  have  room  enough 
for  heavy  fruit  production  for  a  few  years  in  this  rather  "hedge- 
row" planting.     The  following  diagram  illustrates  this  plan. 


p 


X  O  X 

O  Permanent  walnut  trees. 
X  Temporary  walnut  trees. 
P  Filler  trees  of  peaches  or  prunes. 


This  arrangement  would  permit  irrigating  only  a  part  of  the  land 
and  thus  serving  the  water  requirements  of  one  crop  without  affecting 


Bulletin   332]  WALNUT   CULTURE  IN    CALIFORNIA  201 

its  associate  crop,  or  only  slightly.  As  the  trees  grow  older,  the  roots 
become  interwoven  throughout  the  entire  soil  mass  and  gradually  the 
advantages  of  this  plan  lessen,  and  the  filler  trees  should  then  be 
pulled  out. 

FEBTILIZATION 
The  walnut  crop  makes  relatively  small  demands  upon  the  plant 
food  of  the  soil  in  comparison  to  other  fruit  crops.    This  may  be  illus- 
trated by  Table  VII. 

TABLE  VII 

Plant  Food  Elements  Withdrawn  from  the   Soil  by  Various  Fruits, 
Expressed  in  Pounds  per  Acre 

Phosphoric 
Fruit  Amount  of  crop  Nitrogen  acid  Potash 

Oranges    16,300  lbs.  28.23  lbs.  8.63  lbs.  34.39  lbs. 

Lemons   18,900  lbs.  28.53  lbs.  11.52  lbs.  50.84  lbs. 

Apricots    10,000  lbs.  19.73  lbs.  6.40  lbs.  29.00  lbs. 

Walnuts    1,000  lbs.  10.20  lbs.  2.78  lbs.  1.50  lbs. 

Very  little  definite  information  has  been  obtained  on  the  subject 
of  fertilizing  walnuts  since  Smith25  reported  in  1912  the  results  of 
the  fertilizer  trials  on  the  Cudahy  ranch.  In  reviewing  these  results 
the  author  wrote  as  follows : 

The  results  of  this  experiment  are  typical  of  all  experience  in  fertilizing- 
walnuts.  Many  attempts  have  been  made  to  determine  the  most  effective 
practice  in  this  respect,  but  all  of  these  have  shown  no  positive  effect  on 
account  of  variation  in  the  trees  and  an  apparent  lack  of  response  to  fertilizers. 
In  regard  to  walnut  fertilization  it  is,  therefore,  impossible  to  offer  definite 
advice  based  on  actual  experiments,  but  the  most  that  can  be  done  is  to  suggest 
such  practice  as  may  reasonably  be  expected  to  give  good  results  with  any 
crop  on  California  soils.  We  know  in  general  that  nitrogen  and  phosphoric 
acid  are  our  most  needed  elements  and  that  the  application  of  these  substances, 
especially  on  older  plantings,  is  almost  certain  to  result  in  improved  growth 
and  vigor  in  almost  any  plant.  We  also  know  of  the  walnut,  that  individual 
trees  produce  as  a  general  rule,  ip  proportion  to  their  size,  and  that  the  larger 
they  become  the  greater  crops  they  will  bear.  It  is,  therefore,  our  conclusion 
that  in  fertilizing  walnuts,  nitrogen  and  phosphoric  acid  should  be  the  elements 
most  largely  supplied. 

Along  with  fertilization  there  should  not  be  forgotten  the  importance  of 
keeping  the  soil  in  good  mechanical  condition,  especially  in  order  that  it  may 
absorb  and  retain  the  greatest  possible  amount  of  moisture.  Indeed,  it  is 
evident  from  the  results  of  the  Cudahy  experiment,  as  well  as  from  general 
observation,  that  this  factor  is  much  more  important,  at  any  rate  for  several 
years,  than  that  of  fertilization. 

25  Walnut  Culture  in  California,  Univ.  of  Calif.,  Experiment  Station  Bull, 
No.  231,  1912,  p.  188. 


202  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

Field  trials  with  various  kinds  and  amounts  of  fertilizers  applied 
to  several  walnut  groves  are  now  being  carried  on  by  the  University 
of  California,  cooperating  with  the  field  Department  of  the  California 
Walnut  Growers'  Association.  Although  these  trials  have  been  in 
progress  since  the  fall  of  1917,  definite  conclusions  cannot  be  drawn 
from  this  work  at  the  present  writing. 

With  the  widely  varying  soil  conditions  which  exist  in  the  several 
walnut-growing  districts,  a  uniform  fertilizing  program  is  not  likely 
to  be  suitable  to  all.  To  be  profitable,  however,  the  excess  due  to  the 
fertilizers  must  pay  the  added  cost  of  the  fertilizer,  the  labor  of  apply- 
ing, and  the  labor  of  handling  the  increased  crop.  Each  farmer  should 
make  a  careful  study  of  any  fertilizer  treatments  applied  to  his  own 
grove,  in  order  to  be  able  to  judge  of  the  economic  soundness  of 
fertilizing  walnuts  under  his  local  conditions. 

PRUNING 

Unlike  many  of  the  fruit  trees,  such  as  apples,  or  oranges,  the 
pruning  of  the  walnut  tree  has  not  evolved  into  a  very  definite,  sys- 
tematic practice.  In  many  groves  no  pruning  at  all  is  done  except 
the  cutting  off  of  the  lower  limbs  which  interfere  with  cultural 
practice. 

The  fact  that  few  walnuts  are  produced  in  the  centers  of  the  old 
trees,  has  suggested  a  moderate  thinning  out  of  the  branches  from 
year  to  year  in  an  attempt  to  promote  production  more  uniformly 
throughout  the  trees.  Sunlight  is  necessary  for  the  production  and 
maintenance  of  fruit  spurs,  and,  without  some  thinning  out,  prac- 
tically all  of  the  crop  is  produced  on  the  outside  twigs,  in  the  tops, 
and  on  the  sides  of  the  trees.  Some  of  the  most  successful  walnut 
growers  have  adopted  this  sort  of  pruning  as  an  annual  practice, 
cutting  out  the  water  sprouts  and  the  dead  wood;  thinning  out  the 
thickest  growing  portion  of  the  top,  and  reducing  the  number  of 
crossed  limbs  and  weak  crotches.  In  the  absence  of  any  results  of 
systematic  field  trials  this  gradual  and  annual  thinning-out  process 
seems  the  most  advisable. 

This  system  of  pruning  should  not  include  the  heading  back  of 
the  fruiting  limbs,  as  such  a  practice  will  promote  a  rapid  growth  of 
water  shoots  near  the  ends  of  the  parts  which  remain,  defeating  the 
very  attempt  to  shorten  the  limbs.  If  the  limbs  are  too  long  and 
top-heavy  on  the  sides  of  the  trees,  they  should  be  cut  out  entirely, 
or  cut  off  to  lateral  limbs,  thus  restricting  in  a  measure  the  develop- 
ment of  water  shoots. 


Bulletin   332]  WALNUT   CULTURE  IN    CALIFORNIA  203 

In  all  pruning  operations  with  the  walnut,  care  should  be  taken 
to  cut  the  limbs  off  smoothly  with  a  saw  without  leaving  any  stub. 
All  wounds  of  2  inches  in  diameter  or  over  should  be  painted  with  a 
weatherproof  paint.  The  walnut  wood  decays  very  readily  and  there- 
fore any  large  cut  exposed  to  the  weather  for  a  short  period  will 
surely  become  infected  and  start  a  decay  which  will  eventually  spread 
to  the  center  of  the  tree.  The  framework  of  the  tree  thus  undermined 
by  so-called  heart  rot,  may  be  split  by  an  overload  of  nuts,  and  par- 
tially or  wholly  destroyed. 

It  is  expected  that  the  pruning  experiments  now  being  carried 
on  by  the  University  of  California  in  cooperation  with  the  Field 
Department  of  the  California  "Walnut  Growers'  Association  will  event- 
ually give  definite  information  on  this  subject. 


INSECT   AND    DISEASE    PESTS 

Insects. — The  only  insects  which  are  troublesome  to  the  walnut 
industry  are  the  codling  moth,  the  aphis,  and  the  red  spider;  these 
are  mentioned  in  their  probable  order  of  importance. 

Codling  Moth  mid  Aphis. — These  two  insects  may  well  be  con- 
sidered at  the  same  time,  for,  where  they  both  occur,  control  methods 
for  both  may  be  combined  in  one  application. 

The  codling  moth  has  been  known  to  attack  walnuts  in  California 
since  1909,  when  it  was  first  observed  in  the  vicinity  of  Concord.20 
Since  its  first  appearance,  it  has  periodically  become  very  troublesome 
in  several  of  the  walnut  districts  of  southern  California. 

The  injury  caused  by  the  codling  moth  to  walnuts  is  similar  to 
that  seen  in  wormy  apples.  The  worm  eats  a  portion  of  the  kernel 
of  the  nut  and  renders  the  remainder  uneatable.  The  manner  of 
attack  and  entrance  of  the  worm  is  clearly  shown  in  Fig.  27. 

The  aphis  is  a  small  green  plant-louse  which  feeds  on  the  leaves 
and  succulent  growth  by  sucking  their  juices.  This  weakens  the  tree 
at  a  critical  stage,  causes  it  to  produce  less  crop,  lighter  and  smaller 
nuts,  and  imperfect  shells.  The  foliage  of  a  tree  infected  with  this 
insect  becomes  covered  with  ''honey  dew,"  a  sticky  secretion  which 
turns  black  upon  continued  exposure  to  the  air.  The  damage  caused 
by  the  aphis  varies  from  year  to  year.  In  some  sections  the  damage 
is  considerable  every  year,  in  others  only  in  exceptional  years.  The 
aphis  usually  disappear  with  the  approach  of  extremely  hot  weather. 


26  The  Codling  Moth  in  Walnuts.     H.  J.  Quayle,  Monthly  Bull.,  Dept.  Agric. 
California,  vol.  IX,  No.  3,  1920,  p.  64. 


204  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

In  discussing  the  control  of  the  codling*  moth  and  aphis  before 
the  Third  Annual  Convention  of  the  Walnut  Growers  at  Whittier, 
California,  in  January,  1920,  Professor  H.  J.  Quayle  of  the  Citrus 
Experiment  Station  made  the  following  recommendations  :27 

THE  CONTROL  OF  THE  CODLING  MOTH  ALONE 

Where  the  codling  moth  has  been  present  in  orchards  in  past  years  and  the 
amount  of  infestation  has  been  about  5  per  cent  or  greater,  the  trees  should 
be  dusted  or  sprayed  with  basic  arsenate  of  lead  between  May  25  and  June  20. 

1.  Busting. — Five  or  six  pounds  of  dust28  per  tree  is  required  for  trees  of 
medium  size,  and  7  to  10  pounds  for  the  largest  trees.  A  thorough  application 
should  be  made  to  cover  all  the  nuts  on  all  parts  of  the  tree. 

2.  Spraying. — Use  6  pounds  of  dry  basic  arsenate  of  lead  to  a  200-gallon 
spray  tank.  For  trees  of  average  size,  20  to  25  gallons  of  the  spray  per  tree  is 
necessary,  or  %  of  a  pound  per  tree  of  arsenate  of  lead.  For  the  largest  trees, 
30  gallons  per  tree  of  spray  is  necessary,  or  %0  of  a  pound  of  arsenate  of  lead 
per  tree.  All  the  nuts  on  the  trees  should  be  reached  by  the  spray.  It  is  not 
necessary  to  spray  the  trunk  and  branches  or  any  pare  of  the  tree  where  there 
are  no  nuts.  Spray  guns  should  be  used  in  order  to  reach  all  parts  of  the  tree. 
Spraying  will  insure  a  higher  degree  of  control  of  the  codling  moth  than  dust- 
ing, but  the  labor  expense  is  somewhat  more  and  it  takes  longer  to  cover  a 
given  acreage. 

FOR  APHIS  AND  CODLING  MOTH 

Where  past  experience  has  shown  that  the  walnut  aphis,  as  well  as  the 
codling  moth,  may  generally  be  present  in  large  numbers,  use  a  dust  combina- 
tion containing  both  tobacco  and  arsenate  of  lead.  If  spraying  is  to  be  prac- 
ticed instead  of  dusting  for  the  codling  moth,  add  one  pint  of  "black-leaf 
forty ''  to  each  200-gallon  tank  of  arsenate  of  lead. 

FOR  APHIS  ALONE 

Where  there  is  no  codling  moth,  but  the  aphis  is  generally  present  in 
serious  numbers,  use  Nico-dust,  a  material  especially  prepared  for  aphis  control. 
From  2  to  6  pounds  of  this  material  is  necessary  per  tree,  according  to  the 
size  of  tree.  The  best  general  time  for  such  treatment  is  during  the  last  week 
of  May  and  the  first  two  weeks  of  June.  During  years  when  the  aphis  occurs 
in  great  numbers,  it  may  be  advisable  to  dust  a  second  time,  during  July  or 
August. 

RED  SPIDER 

In  some  years  the  red  spider  does  much  damage  to  walnut  trees  in  isolated 
instances.  Injury  is  not  usually  apparent  until  the  middle  of  the  summer, 
when  the  leaves  turn  a  dull  but  decided  yellowish  hue.  A  close  examination 
at  this  time  will  show  the  presence  of  a  countless  number  of  red  spiders,  barely 


*7  From  an  unpublished  manuscript. 

28  Arsenate  of  lead  is  mixed  with  kaolin,  which  acts  as  a  diluent  and  a  carrier. 
Use  only  the  standard  commercial  brands. 


Bulletin  332] 


WALNUT    CULTURE   IN    CALIFORNIA 


205 


visible  to  the  naked  eye.  If  the  injury  goes  on  unchecked,  the  leaves  will  drop 
prematurely,  injuring  the  quality  of  the  current  crop  and  the  development  of 
fruit  buds  for  the  future  crop. 

The  most  practical  control  measure  is  dusting  with  dry  sulfur  and  hydrated 
lime,  three  parts  to  one,  respectively.  Spraying  during  the  dormant  period  has 
not  been  advisable  in  the  past,  because  outbreaks  of  this  insect  have  not  been 
frequent. 


Fig.  27.      (From  "The  California  Walnut.") 

(a)  Appearance  of  green  nut  after  worm  has  entered  on  open  side,  (b)  Codling 
moth  worm  usually  enters  green  nut  on  the  stem  end  as  here  shown,  (c)  A  round 
or  oval  opening  on  the  stem  end  of  the  cured  nut  is  almost  a  sure  sign  that  the 
worm  has  paid  his  respects  inside,  (d)  Section  of  walnut  showing  the  codling  moth 
worm. 


WALNUT  BLIGHT 

The  walnut  blight  is  by  far  the  most  destructive  disease  affecting 
the  walnut  crop.  The  prevalence  of  this  disease,  as  previously  stated, 
varies  considerably  from  year  to  year,  and  is  usually  much  worse  in 
the  foggy  coastal  districts  than  in  the  inland  valleys.     In  years  of 


206  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

bad  outbreaks,  the  blight  has  probably  destroyed  from  15  per  cent  to 
20  per  cent  of  the  crop.  It  is  a  bacterial  disease  which  attacks  the 
young  and  tender  growth  and  spreads  to  the  more  mature  wood, 
causing  the  affected  areas  to  turn  black  and  die.  Under  favoring 
conditions,  the  disease  is  especially  destructive  to  the  nuts.  If  it 
attacks  the  nuts  early,  it  causes  them  to  turn  black  and  drop  off  when 
one-eighth  to  one-half  of  an  inch  in  diameter.  It  causes  mature  nuts 
to  remain  undeveloped,  and  may  make  full-grown  nuts  unmarketable, 
except  as  culls.  It  shows  on  the  nut  as  black  spots  most  prevalent  at 
the  calyx  end,  but  often  scattered  over  its  entire  surface. 

At  present  there  is  no  control  known  for  this  disease.  Attempts 
to  lessen  the  prevalence  of  the  blight  by  spraying  and  by  the  com- 
bination of  spraying  and  pruning  off  the  old  blight  cankers  from  the 
twig  growths,  have  had  no  measurable  effect  upon  the  disease.29 

Some  of  the  varieties  now  being  propagated  are  more  resistant  to 
blight  than  the  average  seedling  tree.  At  present  the  greatest  likeli- 
hood of  relief  from  this  disease  lies  in  securing  resistant  or  immune 
varieties. 

MELAXUMA 

This  troublesome  disease  of  the  walnut  tree  has  occurred  in  isolated 
and  serious  outbreaks  in  Santa  Barbara,  Ventura,  Los  Angeles,  and 
Orange  counties.  Careful  studies  of  this  disease  were  made  and 
reported  in  1914  to  1915  by  Fawcett.30  The  nature  and  treatment 
of  the  trouble  may  best  be  summarized  by  quoting  from  the  above 
author.  (For  a  detailed  discussion  of  the  disease,  see  the  original 
publication.) 

Because  of  the  oozing  of  dark  watery  material  to  the  surface  of  the 
affected  areas,  this  disease  is  often  confused,  under  the  name  of  "black  sap," 
with  sunburn,  frost  injury,  injuries  to  the  bark  in  cultivation,  injury  from  the 
decay  of  wood  at  places  where  limbs  have  been  cut  off,  and  other  troubles  in 
which  a  "black  sap"  may  ooze  out  during  the  active  growing  period  of  the 
tree.  It  should  not  be  taken  for  granted,  therefore,  that,  because  a  black  ooze 
is  seen  on  the  trunk  or  larger  limbs  of  a  walnut  tree,  Melaxuma  is  necessarily 
present. 

As  the  term  "black  sap"  has  already  been  used  by  E.  E.  Smith  to  designate 
a  result  of  sunburning,  this  name  was  not  considered  suitable  for  the  disease 
here  being  considered.  The  word  Melaxuma,  derived  from  two  Greek  words 
meaning  "black"  and  "juice"  was  therefore  adopted,  and  is  now  in  fairly 
common  use  to  designate  the  disease  herein  described. 


29  An  Attempt  to  Control  Walnut  Blight.  H.  S.  Fawcett  and  L.  D.  Batchelor. 
Monthly  Bulletin,  California  Dept.  Agri.,  vol.  IX,  No.  5-6.     1920. 

so  Melaxuma  of  the  Walnut,  H.  S.  Fawcett,  Univ.  of  Calif.  Experiment  Station 
Bull.  No.  261,  1915. 


Bulletin   332]  WALNUT   CULTURE  IN   CALIFORNIA  207 


Description  of  the  Disease 

Melaxuma  shows  its  effect  most  strikingly  during  the  summer  after  the 
growth  is  well  started.  On  trees  severely  affected  at  this  time,  there  will  be 
seen  large  black  sunken  cankers  on  the  trunk  and  larger  limbs  and  often  a 
sudden  wilting  of  the  smaller  limbs  and  twigs.  This  sudden  wilting  of  the 
smaller  limbs  is  so  different  from  the  effects  of  walnut  blight  or  bacteriosis, 
that  it  is  easily  distinguished  from  that  disease. 

The  most  common  location  for  Melaxuma  cankers  is  at  the  crotch  of  the 
tree  where  the  first  limbs  join  the  trunk.  The  first  evidence  of  the  disease 
is  often  a  black  area  on  the  otherwise  grayish  bark  which  looks  like  a  dab  of 
tar,  as  shown  by  Fig.  28.  This  is  due  to  the  staining  of  the  bark  by  a  black 
watery  substance  that  forms  under  it.  The  diseased  area  later  becomes  slightly 
sunken,  shrinks,  and  cracks.  The  ' '  black  sap ' '  then  oozes  out  in  considerable 
quantities  and  stains  the  bark  as  it  runs  down  the  limb  or  trunk.  The  wood 
underneath  is  discolored  for  a  short  distance  and  this  discoloration  usually 
extends  beyond  the  margin  of  the  killed  bark.  The  diseased  areas,  as  a  rule, 
do  not  extend  entirely  around  a  limb  in  one  season,  but  affect  only  about  one- 
third  or  one-half  the  circumference  of  the  bark.  Later  in  the  summer  or  fall 
the  increase  in  the  size  of  the  areas  is  slower  and  often  appears  to  be  stopped. 
One  part  of  the  margin  of  the  canker  may  dry  out  and  begin  to  heal  over,  while 
the  other  continues  to  advance  or  remains  stationary  until  the  next  spring,  when 
it  begins  to  advance  rapidly  again.  A  few  cankers  on  large  limbs  of  vigorous 
trees  may  even  heal  over  without  treatment.  More  often  there  is  an  enlarge- 
ment from  year  to  year  which  in  two  or  more  years  extends  entirely  around  a 
large  limb,  causing  it  to  wilt  and  die.  These  limbs  occasionally  wilt  suddenly, 
the  dried-up  nuts  and  leaves  remaining  attached  for  some  time.  When  cankers 
occur  on  the  trunks  they  generally  follow  some  injury  to  the  bark  made  by  a 
plow,  cultivator  or  other  instruments  used  in  the  cultivation   of  the  orchard. 

From  the  observations  and  results  of  experiments  so  far  made,  the  following 
tentative  treatment  is  suggested: 

Cut  out  the  cankers  that  have  not  gone  too  far  on  the  trunk  and  larger 
limbs  and  disinfect  the  wounds  thus  made.  (See  Fig.  29.)  The  dead  and 
discolored  bark  should  be  cut  away,  getting  a  little  beyond  the  margin  of  dead 
tissue.  If  the  cankers  are  not  large  and  the  wood  underneath  has  not  been 
stained  deeply,  it  will  pay  to  dissect  all  the  discolored  wood  as  well  as  the 
bark.  Probably  one  of  the  best  disinfectants  to  apply  to  the  wound  is  the 
Bordeaux  paste,31  that  is  recommended  for  lemon  gummosis.     If  the  canker  is 


si  Bordeaux  Paste.  The  formula  for  Bordeaux  paste  is  as  follows:  12  pounds 
of  Milestone  (copper  sulfate)  dissolved  in  8  gallons  of  water  in  a  wooden,  earthern 
or  glass  vessel;  and  24  pounds  of  quick-lime  slaked  in  8  gallons  of  water.  When 
the  lime  is  cool,  stir  together  about  equal  parts  by  volume  of  each,  for  making 
enough  mixture  to  last  for  one  day  only.  The  Milestone  is  most  easily  dissolved 
by  suspending  it  in  a  sack  at  the  top  of  the  water  over  night.  If  the  Milestone  is 
pulverized  and  suspended  in  warm  water,  it  dissolves  rapidly.  Good  lime  that  is 
not  air-slaked  should  be  used,  and  after  slaking  it  with  the  water,  it  should  be 
allowed  to  cool  before  being  used  in  making  paste.  If  covered  to  avoid  evapora- 
tion each  ingredient  will  keep  indefinitely,  but  after  mixing,  the  paste  slowly 
deteriorates.  Where  it  is  being  used  over  a  number  of  days  or  weeks,  just  enough 
of  the  wet  slaked  lime  and  the  Milestone  solution  should  be  mixed  to  make  paste 
enough  to  last  for  one  day,  leaving  the  remainder  unmixed  in  separate  vessels.  It 
may  be  applied  with  large  brushes,  as  in  whitewash. 


208  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

large  and  has  been  in  the  tree  a  long  time,  the  wood  may  be  stained  so  deeply 
as  to  render  the  work  of  cutting  out  all  discolored  wood  too  expensive.  If  the 
canker  has  practically  girdled  the  limb,  the  limb  had  better  be  cut  out. 

WINTER  INJURY   OR  DIE-BACK 
This  trouble  of  the  walnut  tree  has  been  made  the  subject  of  con- 
siderable study  by  Smith32  and  Batchelor  and  Reed.33     Perhaps  it 
can  be  summarized  best  at  this  time  by  quoting  from  the  last  men- 
tioned authors. 

Winter  injury  or  die-back  of  walnuts  is  characterized  by  a  sudden  death 
of  the  tops  of  the  trees.  Such  injury  is  usually  first  noticeable  during  the 
early  spring  following  the  dormant  period. 

The  most  common  causes  of  the  winter  injury  or  die-back  are  given  below. 

1.  Early  autumn  frosts  kill  the  immature,  growing  shoots.  Young  walnut 
trees  are  more  subject  to  injury  from  this  cause  than  older  trees,  because  they 
are  usually  later  in  maturing  their  new  wood.  Such  frosts  cause  the  foliage 
to  drop  prematurely  and  injure  the  growing  tips  of  the  twigs.  The  denuded 
twigs  are  subject  to  further  injury  from  subsequent  fall  and  winter  sunburn. 
The  presence  or  extent  of  this  injury  is  usually  overlooked  until  the  following 
spring. 

To  reduce  the  danger  from  autumn  frosts  it  is  advisable  to  withhold  the 
late  summer  irrigation  in  order  to  promote   the   early  maturity   of  the  trees. 

2.  Winter  drought  causes  die-back  in  either  young  or  bearing  walnut  groves. 
Trees  suffering  from  this  condition  fail  to  make  new  growth  in  the  spring, 
except  from  the  trunks  or  main  limbs.  The  new  growth  on  such  trees  has  lost 
so  much  water  during  the  winter  that  the  buds  are  unable  to  develop  in 
spring.  The  cause  of  the  die-back  has  been  found  to  be  due  to  an  extremely 
low  moisture  content  of  the  soil  during  a  large  part  of  the  winter.  Under  such 
conditions,  the  water  lost  from  the  young  shoots  during  the  winter  cannot  be 
replenished  by  the  root  system,  and  the  shoots  die  from  desiccation. 

Fall  and  winter  irrigation  of  the  walnut  groves  has  been  found  to  eliminate 
the  injury  from  winter  drought.  The  amount  of  irrigation  will  depend  upon 
the  type  of  soil,  the  amount  of  soil  moisture  present  at  the  end  of  the  harvest 
season,  and  the  rainfall  which  may  be  expected  later  in  the  season. 

Figure  30  shows  a  walnut  tree  thus  affected  and  afterward  cured 
by  winter  irrigation. 

The  late  fall  or  early  winter  irrigation  of  the  walnut  groves, 
especially  in  the  inland  valleys,  may  be  looked  upon  as  an  insurance 
policy  against  this  form  of  die-back.  To  be  effective  it  should  be 
applied  in  December  and  January.  In  some  years  subsequent  rains 
will  show  that  the  die-back  insurance  policy  was  unnecessary.     The 


32  R.  E.  Smith,  Walnut  Culture  in  California,  Univ.  Calif.  Experiment  Station 
Bull.  No.  231,  1912,  p.  372. 

33  L.  D.  Batchelor  and  H.  S.  Reed,  Winter  Injury  or  Die-back  of  the  Walnut, 
Univ.  of  Calif.  Experiment  Station  Circ.  No.  216,  1919. 


Bulletin  332 


WALNUT    CULTURE   IN    CALIFORNIA 


209 


Fig.  28. — One  of  the  first  evidences  of  Melaxuma.  The  black  sappy  ooze  has 
appeared  at  the  outer  edges  of  the  killed  bark  at  the  crotch.  The  bark  between 
has  been  killed  and  pycnidia  of  Dothiorella  have  already  appeared  on  the  surface 
of  part  of  the  killed  bark.     (From  Fawcett,  Bulletin  2 til.) 


210  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

same  apparent  objection,  however,  applies  to  fire  insurance  on  the 
barn  or  house,  during  the  years  that  there  are  no  fires  on  the  property. 

3.  A  high  water-table  may  be  a  contributing  factor  in  die-back.  A  perma- 
nently high  water-table  causes  the  trees  to  prolong  their  growing  season,  with 
the  result  that  they  are  killed  by  frosts.  The  sudden  rise  of  a  fluctuating  water- 
table  may  kill  a  large  part  of  the  root  system  and  produce  a  typical  die-back 
in  the  tops,  even  though  the  wood  is  mature. 

4.  Alkali  soils  containing  such  a  high  salt  content  as  to  injure  the  root 
systems  of  walnut  trees,  also  cause  the  tops  of  the  trees  to  die  back  in  response 
to  the  root  injury.  In  the  initial  stages  of  alkali  injury,  the  leaves  turn  brown 
at  the  margin  and  fall  prematurely.  The  denuded  shoots  sometimes  put  out  a 
new  set  of  leaves  in  the  fall.  The  top  of  the  tree  gradually  dies  back  until 
the  entire  tree  is  lost. 

If  injury  is  being  caused  by  irrigation  water  containing  too  much 
alkali,  obviously  the  use  of  such  water  should  be  discontinued. 


HARVESTING,    CURING,    PACKING,    AND    COST    OF    PRODUCTION 

HARVESTING 

During  favorable  seasons  and  in  well-cared-for  groves  the  husks 
of  the  nuts  crack  open  and  adhere  temporarily  to  the  twigs,  while  the 
nuts  drop  clean-shelled  to  the  ground.  This  natural  dropping  of  the 
great  bulk  of  the  nuts  occurs  with  most  varieties  between  September  1 
and  November  7.  There  are  usually  enough  nuts  on  the  ground  to 
justify  the  first  picking  by  the  second  week  in  September.  The 
natural  falling  of  the  nuts  is  hastened  by  the  shaking  of  the  trees 
by  means  of  long  poles  with  hooks  attached  to  the  ends. 

During  the  harvest  period  the  nuts  are  picked  up  three  or  four 
times  before  the  total  crop  has  matured  and  dropped.  It  is  not  con- 
sidered good  practice  to  allow  the  nuts  to  remain  for  a  long  time  on 
the  ground.  Nuts  thus  neglected  are  subject  to  the  work  of  ants  in 
the  kernels,  and  may  be  rained  on  by  the  autumn  showers.  The 
effect  of  rain  upon  the  nuts  may  be  entirely  superficial  if  they  are 
picked  up  promptly  after  they  are  dry  again.  At  the  best,  however, 
they  will  be  dirty  and  more  or  less  stained,  causing  extra  work  of 
washing  in  preparing  them  for  the  packing  house,  and  making  proper 
bleaching  more  difficult.  Nuts  long  neglected  on  the  ground  after  a 
rain  become  mouldy  and  stained  on  the  outside  of  the  shells,  especially 
those  which  have  a  portion  of  the  husk  adhering  to  them.  If  the 
stain  of  the  mould  is  pronounced,  it  will  be  impossible  to  bleach  it, 
and  the  nut  must  be  graded  as  a  cull.  If  further  neglected,  the  mould 
from  the  outside  of  the  nut  may  spread  to  the  kernel,  through  the 
base  of  the  nut,  and  thus  lower  the  value  even  as  a  cull  nut. 


Bulletin  332] 


WALNUT    CULTURE   IN    CALIFORNIA 


211 


A  certain  percentage  of  the  nuts  will  drop  with  the  husks  adhering 
to  them.  These  are  commonly  known  as  ' '  stick-tights ' '  and  are  likely 
to  be  inferior  to  the  clean  shelled  nuts  in  their  plumpness  and  in  the 
appearance  of  the  kernels.     There  is  usually  a  high  percentage  of 


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Fig.  29. — Walnut  trees  showing  the  location  of  Melaxuma  cankers  that  had 
been  cut  out  and  treated  with  Bordeaux  paste  the  year  previous.  Notice  the  rows 
of  holes  in  the  bark  of  one  tree,  made  by  sapsuckers.  These  probably  served  in 
this  case  as  a  partial  means  of  infection.     (Fawcett,  H.  S.,  Bulletin  261.) 

blanks,  shriveled,  mouldy,  and  dark-colored  kernels  among  the  stick- 
tight  nuts.  The  percentage  of  stick-tights  is  greater  during  seasons 
of  abnormally  high  temperatures,  when  the  nuts  are  sunburned  and 
when  the  husks  are  affected  with  blight.     Again,  trees  which  suffer 


212  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

from  drought  during  the  latter  part  of  the  growing  period,  or  which 
are  subject  to  the  attacks  of  aphis,  red  spider,  or  for  any  other  reason 
lose  their  leaves  prematurely,  produce  a  high  percentage  of  stick- 
tights  and  inferior  nuts. 

In  general,  the  stick-tight  nuts  are  husked  by  hand,  especially  those 
on  which  the  husks  are  actually  green,  although  several  large  ranches 
have  made  devices  to  husk  them  by  machinery.  The  portions  of  the 
husks  which  have  dried  on  the  nuts,  as  in  the  case  of  sunburning,  and 
blighted  husks,  are  removed  by  the  washing  machine,  described  later. 

The  nuts  are  usually  picked  up  and  sacked  by  Mexican  and  Jap- 
anese families  who  contract  to  perform  this  work. 

WASHING  AND  CUEING 

After  being  picked  up  and  sacked,  the  nuts  are  ready  for  curing. 
If  the  majority  of  them  are  dirty  from  lying  on  the  ground  during 
showers,  it  is  necessary  to  wash  them.  This  washing  is  done  in  large 
cylindrical  drums  made  of  coarse  wire  netting,  in  which  the  nuts  are 
slowly  revolved  under  a  stream  of  water,  grinding  against  each  other 
and  against  the  wires  which  form  the  sides  of  the  drum.  They  are 
thus  cleansed  of  all  loose  foreign  material.  This  washing  process  will 
not  remove  the  stains  on  the  shells  due  to  sunburned  and  blighted 
husks  sticking  to  the  nuts.  There  is  practically  no  need  for  washing 
the  clean-shelled  nuts. 

The  curing  is  accomplished  by  spreading  the  nuts  out  in  shallow 
trays,  with  bottoms  made  of  slats  spaced  about  one-half  an  inch 
apart.  The  nuts  should  not  be  left  exposed  to  the  sun  during  the 
entire  day  if  the  weather  is  especially  clear  and  hot,  for  the  drying 
will  be  so  rapid  that  many  of  the  nuts  will  crack  open.  If  the  trays 
are  spread  out  in  the  morning  and  the  nuts  thoroughly  stirred  several 
times,  the  trays  may  be  piled  up  when  the  nuts  are  well  warmed  up. 
The  slow  drying  which  goes  on  while  the  trays  are  in  piles  of  course 
prolongs  the  length  of  the  process,  but  is  good  insurance  against  the 
splitting  of  poorly  sealed  nuts  and  their  rejection  as  culls  by  the 
packers.  Figure  31  shows  a  yard  full  of  trays  ready  to  be  piled  up 
after  being  thoroughly  warmed  up  by  the  morning  sun.  The  trays 
should  be  so  piled  up  as  to  allow  ventilation  between  them. 

If  the  walnut  grove  is  large  enough,  it  will  save  expense  to  use 
a  "drying  house"  for  the  curing  process.  Such  "dry  houses"  are 
built  with  outer  walls  of  lath  to  give  good  ventilation  as  shown  by 
Fig.  32.  The  drying  bins  are  arranged  one  above  another.  The  nuts 
are  carried  away  from  the  washing  machine  by  a  belt  and  thence 


Bulletin  332] 


WALNUT    CULTURE   IN    CALIFORNIA 


213 


m  ■      .$ht+ 


^r-^m^^^, ^   ~;  "*» 


Fig.   30. — Showing  the  effect  of  winter  drouth  and  recovery  of  the  tree  two 
years  later  after  irrigation. 


Fig.  31. — Drying  walnuts  by  means  of  spreading  them  on  trays  placed  on  low 
racks. 


214  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

elevated  to  the  upper  bin,  as  shown  in  Fig.  33.  The  nuts  are  dumped 
mechanically  from  one  bin  to  the  bin  below  it.  Thus  the  nuts  work 
through  the  whole  series  of  bins  from  the  top  down.  There  may  be 
seven  or  eight  of  these  bins  and  by  dumping  them  each  once  a  day, 
the  nuts  will  be  passed  through  the  house  in  seven  or  eight  days.  As 
the  nuts  leave  the  bins  after  this  period,  they  are  usually  sufficiently 
cured  to  be  ready  to  go  to  the  packing  house,  to  be  bleached,  graded, 
and  bagged  for  shipment.  If  the  weather  is  warm  and  the  atmosphere 
very  dry,  the  nuts  may  pass  through  the  drying  house  in  four  or  five 
days.  Much  will  depend  also  upon  the  condition  of  the  nuts  when 
they  go  into  the  house,  as  well  as  upon  the  exposure  of  the  house  to 
breezes,  sunshine,  etc. 

As  the  nuts  pass  along  a  belt  when  they  come  from  the  drying  bins, 
or  as  they  are  sacked  from  the  trays,  they  should  be  carefully  sorted 
to  pick  out  the  culls.  Nuts  are  thrown  into  this  cull  class  if  they  are 
cracked,  perforated,  badly  stained  by  blight  or  sunburn,  wormy,  or 
if  a  part  of  the  shuck  is  adhering  to  the  shell.  The  good  nuts  are  now 
ready  for  the  packing  house,  while  the  culls  are  kept  separate  to  go 
finally  to  the  cracking  plant. 

PACKING 

Although  the  packing  and  selling  of  the  walnut  crop  in  California 
is  quite  distinct  from  its  production,  it  is  nevertheless  desirable  for 
each  grower  to  know  how  his  nuts  are  to  be  graded  and  packed,  in 
order  that  he  may  realize  more  fully  the  importance  of  delivering 
high-grade  nuts  to  the  packing  house.  About  85  per  cent  of  the  walnut 
crop  of  California  is  packed  and  sold  through  the  local  houses  which 
are  affiliated  with  the  California  Walnut  Growers'  Association.  It 
may  therefore  be  proper  to  relate  briefly  the  processes  followed  in 
these  houses  in  preparing  the  crop  for  shipment.  Mr.  W.  T.  Webber 
has  clearly  described  the  stages  of  grading  and  packing  in  the  words 
quoted  below. 

ELIMINATING  IMPERFECT  NUTSs* 

After  being  properly  cured,  the  nuts  are  delivered  to  the  local  packing  house 
by  the  grower,  where  they  are  first  run  over  a  rough  screen  which  frees  them 
from  dirt  and  all  foreign  matter.  From  this  screen  the  nuts  pass  through  a 
suction  machine,  a  device  which  lifts  the  blank  or  imperfectly  filled  nuts  over 
a  trap  and  allows  the  full-meated  nuts  to  pass  through.  From  the  suction 
machine  the  full-meated  nuts  pass  on  to  an  endless  belt  where  girl  cullers, 
seated  on  each  side,  remove  the  ill-shapen  and  bad-appearing  nuts. 


34  The  California  Walnut,  1919,  pp.  47-57. 


Bulletin  332] 


WALNUT    CULTURE   IN    CALIFORNIA 


215 


Fig.   32. — "Drying  houses''  are  built  with  outer  walls  of  lath  to  give  good 
ventilation. 


Fig.  33. — The  nuts  are  carried  away  from  the  washing  machine  (shown  in  the 
right  background)  by  a  belt,  passing  before  the  inspector  who  removes  the  culls. 
Thence  they  are  carried  by  the  elevator,  shown  in  the  center,  to  the  bins  shown  at 
the  right  center  of  illustration. 


216  UNIVERSITY   OF    CALIFORNIA — EXPERIMENT    STATION 

BLEACHING 

The  next  treatment  is  that  of  bleaching.  The  nuts  are  given  a  bath  of 
from  one  to  two  minutes'  duration  by  passing  them  through  large  drums  par- 
tially filled  with  a  liquid  bleaching  solution,  for  the  purpose  of  removing  dirt 
and  stain  and  brightening  the  appearance  of  the  shells. 

From  the  bleaching  drums  they  are  passed  over  another  shaker  which  frees 
them  from  accumulated  drops  of  the  solution,  and  they  are  then  elevated  and 
graded  to  size. 

GRADING 

For  this  process  large  galvanized-iron  cylindrical  graders  are  used.  Each 
grader  is  10  feet  long  and  40  inches  in  diameter,  set  on  a  6-inch  pitch  and  has 
a  capacity  to  properly  grade  one  and  one-half  tons  of  walnuts  per  hour.  A 
soft-shell  walnut  grader  contains  somewhat  over  8000  li£2  incn  square  openings 
and  the  walnuts  which  pass  over  the  grader  without  falling  out  at  one  of  these 
openings  are  known  as  No.  1  Grade.  Those  which  fall  out  comprise  the  No.  2 
Grade. 

Budded  graders  contain  somewhat  less  than  8000  1%6  inch  square  openings, 
and  the  nuts  passing  over  constitute  the  Fancy  Grade  and  those  falling  through 
the  Standard  Grade. 

From  the  grader  the  walnuts  again  pass  on  to  a  grading-belt,  where  those 
that  have  not  been  bleached  properly,  or  have  been  broken  by  the  bleaching 
and  grading  operations,  are  removed.  The  perfect  nuts  then  go  into  the  drying- 
bins,  and  after  a  period  of  from  24  to  48  hours  are  packed,  100  pounds  net 
weight,  in  burlap  bags,  ready  for  shipment. 

THE  STANDARD  FOR  FIRST-GRADE  NUTS 
A  nut  is  considered  satisfactory  if  its  size  is  in  accordance  with  the  speci- 
fications for  the  No.  1  or  the  No.  2  grade;  if  its  kernel  is  plump  and  sound, 
not  too  dark  in  color;  if  it  is  not  wormy,  mouldy,  or  rancid;  and  if  one  portion 
of  the  shell  has  not  split  and  fallen  away  from  the  other.  The  shell  must  have 
no  material  outward  blemish  and  must  show  a  clean  bright  color. 

In  most  seasons  the  California  Walnut  Growers'  Association  guar- 
antee that  at  least  90  per  cent  of  the  nuts  in  each  sack  sold  by  them 
comply  with  the  above  standard. 

SELLING  THE  CROP 
As  noted  heretofore,  about  85  per  cent  of  the  walnut  crop  is  sold 
by  the  California  Walnut  Growers'  Association.  This  central  organ- 
ization is  a  non-capital,  non-profit,  cooperative  association,  composed 
of  approximately  forty  local  packing  associations.  The  local  associa- 
tions are  also  organized  on  the  cooperative  principles.  The  local 
plants  grade  and  pack  the  nuts  of  their  members  in  accordance  with 
the  standard  agreed  upon  by  the  central  association.  The  central 
association,  by  a  rigid  inspection,  maintains  the  standards  set  and 
performs  its  prime  function  of  selling  the  crop.  The  walnut  growers, 
affiliated  as  above  noted,  thus  receive  the  actual  selling  price  of  their 
nuts  minus  the  bare  cost  of  grading,  packing,  and  selling  the  nuts. 


Bulletin   332]  WALNUT   CULTURE  IN   CALIFORNIA  217 

The  walnut  growers  who  do  not  belong  to  the  cooperative  associa- 
tions usually  sell  their  crops  to  the  various  independent  packing  firms, 
who  later  grade  and  pack  the  nuts  according  to  their  own  standards. 

INCOME    TO    BE    DERIVED    FROM    CALIFORNIA    WALNUTS 

In  order  to  assist  the  beginner  in  considering  the  probable  profits 
to  be  derived  from  growing  walnuts  the  following  table  has  been 
prepared. 

The  average  yield  for  the  state  varies  from  year  to  year.  Numer- 
ous statistical  publications  indicate  that  a  ten-year  average  yield  for 
bearing  orchards  is  approximately  800  pounds  per  acre.  It  follows, 
therefore,  if  an  orchard  is  picked  at  random  the  chances  are  even 
that  the  average  yield  for  ten  years  is  800  pounds  or  less.  This  figure 
has  been  used  as  a  conservative  estimate  for  business  purposes  in  the 
calculations  below. 

Goodspeed35  found  from  a  survey  of  nearly  a  thousand  groves  that 
the  cost  of  operation  of  a  bearing  grove  was  approximately  $50.00 
per  acre  under  conditions  similar  to  those  set  forth  in  the  following 
table,  exclusive  of  depreciation.  This  figure  has  been  increased 
slightly  from  the  above  mentioned  calculations  in  order  to  cover  depre- 
ciation and,  in  the  especially  heavy  producing  groves,  to  also  cover 
the  extra  cost  of  harvesting  big  crops. 

Hunt30  writes  that,  "A  competent  farmer  should  expect,  and  no 
one  should  undertake  to  farm  unless  he  may  reasonably  expect,  to 
produce  50  per  cent  more  than  the  average.  On  this  basis,  the  gross 
income  per  annum  may  be  estimated  at  25  per  cent  of  the  capital 
invested.  This  does  not  mean  that  every  legitimate  farm  enterprise 
will,  or  should  bring  in  just  25  per  cent  of  the  capital  invested.  This 
statement  is  only  meant  to  give  one  a  'yard  stick'  with  which  to 
measure  any  definite  farming  enterprise." 

The  average  annual  gross  income  from  walnuts,  as  shown  by  the 
preceding  table,  amounts  to  approximately  19  per  cent  of  the  capital 
invested.  This  does  not  necessarily  mean  that  the  table  indicates 
an  inflated  capitalization  of  the  industry,  because  this  return  has 
attracted  adequate  capital  to  the  walnut  industry  in  competition  with 
general  farming,  owing  to  the  facts  that  walnuts  have  required  less 
investment  in  farm  machinery  than  many  other  crops;  the  trees  are 
long-lived,  and  therefore  there  is  a  smaller  annual  depreciation  than 

35  "The  California  Walnut,"  published  by  California  Walnut  Growers'  Asso- 
ciation, 1919,  p.  20. 

so  ' '  Suggestions  to  the  Settler  in  California, "  T.  F.  Hunt,  Univ.  of  Calif. 
Experiment  Station  Circ.  No.   210,  1919,  p.  4. 


218 


UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 


with  many  fruit  crops  and  animal  industries;  walnuts  are  less  perish- 
able during  the  harvesting,  packing,  and  marketing  operations  than 
many  crops ;  and  the  annual  prices  of  walnuts  received  by  the  growers 
have  gradually  increased  during  the  past  twelve  years,  without  any 
great  speculative  fluctuations  from  year  to  year. 


Estimate  of  the  Average  Returns   which   may  be  Expected  from 
California  Walnut  Groves 


Yield  per  acre. 


Safe  estimate] 
for  business  " 
purposes 

800  lbs. 

(approximate 
average  in  California) 
$700.00 

130.98 


Income  which 
competent 
ggTmen  may  hope 
to  obtain 

1200  lbs. 


Possible, 
but 
extra- 
ordinary 

2000  lbs. 


Valuation  per  acre $700.00  $1000.00  $1700.00 

Gross  income  per  acre1 130.96  196.44  327.40 

Annual    cost    of    operation 

per  acre2 $55.00  $60.00  $70.00 

Annual  cost  of  packing  and 

marketing,  per  acre 16.00  24.00  40.00 

Total  charge  per  acre  against 

crop $  71.00  $  84.00  $110.00 

Net  income  per  acre3 $59.96  $112.44  $217.40 

Interest  at  4%  on  valuation4  28.00  40.00  68.00 

Profit  per  acre5..., $31.96  $72.44  $149.40 

(Above  what  might  have  been 
realized,  with  the  capital  in- 
vested in  safe  securities  and  the 
owner  working  out  for  wages). 

1  Based  on  selling  price  of  No.  1  nuts  for  ten  years,  1907-1918  (16.37  cents). 
All  nuts  would  not  be  No.  1,  therefore,  the  above  returns  are  slightly  exaggerated. 

2  Includes  irrigation,  pruning,  cultivation,  harvesting,  depreciation  and  taxes. 

3  Gross  returns  minus  all  expenses. 

4  The  rate  of  4  per  cent  is  used  here  to  charge  against  the  grove  as  the  return 
which  might  have  been  made  had  the  owner  invested  in  securities  rather  than  a 
walnut  grove.  This  rate  of  interest  is  in  harmony  with  investments  made  with 
a.  minimum  of  risk  and  a  minimum  of  skill  and  effort  on  the  part  of  the  investor. 

5  The  gross  returns  minus  all  expenses  and  4  per  cent  of  the  valuation  of  the 
property. 


